The Science Book

EXPANDING HORIZONS 99

See also: Isaac Newton 62–69 ■ Louis Agassiz 128–29 ■ Charles Darwin 142–49 ■ Marie Curie 190–95 ■
Ernest Rutherford 206–13

older, since eons of time would its parts,” with a new world activity. Today, this is known as
be needed for chalk mountains to constantly reshaped and recycled the geological cycle. From this
build up from the remains of marine from the ruins of the old. evidence, Hutton declared that
fossils, but he did not want to all continents are formed from
publish this view without evidence. Hutton formulated his Earth- materials derived from previous
machine theory before he had found continents by the same processes,
Secrets of the rocks the supporting evidence, but, in and that these processes still
In Scotland, quite a different 1787, he found the “unconformities” operate today. Famously, he wrote
approach to the problem of Earth’s he was looking for—breaks in the that “the result, therefore, of this
age was being taken by James continuity of sedimentary rocks. present enquiry is, that we find
Hutton, one of the preeminent He saw that much of the land had no vestige of a beginning—no
natural philosophers of the Scottish once been seabed, where layers prospect of an end.”
Enlightenment. Hutton was a of sediment had been laid down and
pioneer of geological fieldwork, and compressed. In many places these The popularization of Hutton’s
used field evidence to demonstrate layers had been pushed upward, so ideas about “deep time” was
his arguments to the Royal Society that they were above sea level, and primarily due to John Playfair, a
of Edinburgh in 1785. often distorted, so that they were Scottish scientist who published
not horizontal. He repeatedly found Hutton’s observations in an
Hutton was impressed by that rock material from the illustrated book, and to British
the apparent continuity of the truncated upper boundary of older geologist Charles Lyell, who
processes by which landscape was strata was incorporated into the transformed Hutton’s ideas into a
denuded and its debris deposited base of the younger rocks above. system called uniformitarianism.
into the sea. And yet all these This held that the laws of nature ❯❯
processes did not lead to loss of the Such unconformities showed
land surface, as might be expected. that there had been many episodes In 1770, Hutton built a house
Perhaps thinking of the famous in Earth’s history when the overlooking Salisbury Crags in
steam engine built by his friend sequence of erosion, transportation, Edinburgh, Scotland. Among the
James Watt, Hutton saw Earth as and deposition of rock debris had crags he found evidence of volcanic
“a material machine moving in all been repeated, and when rock penetration through sedimentary rock.
strata had been moved by volcanic

100 JAMES HUTTON

have always been the same, and The mind seemed to took Earth’s initial temperature at
therefore the clues to the past lie grow giddy by looking so 7,000°F (3,900°C) and applied the
in the present. However, while far into the abyss of time. observation that temperature
Hutton’s insights concerning the increases as you go downward from
antiquity of the planet rang true John Playfair the surface—by about 1°F (0.5°C)
to geologists, there was still no over every 50 ft (15 m) or so. From
satisfactory method of determining observed that the processes of this, Kelvin calculated that it had
just how old the planet was. erosion and deposition of the rock taken 98 million years for Earth to
materials that make up such strata cool to its present state, which he
An experimental approach were very slow—estimated to be later reduced to 40 million years.
Since the end of the 18th century, a few inches (centimeters) every
scientists had recognized that 100 years. In 1858, Charles Darwin A radioactive “clock”
Earth’s crust comprises successive made a somewhat ill-judged foray Such was Kelvin’s prestige that
layers of sedimentary strata. into the debate when he estimated his measure was accepted by most
Geological mapping of these strata that it had taken some 300 million scientists. Geologists, however,
revealed that cumulatively they are years for erosion to cut through the were left feeling that 40 million
very thick and many contain the Tertiary and Cretaceous period years was simply not long enough
fossil remains of the organisms rocks of the Weald in southern for the observed rates of geological
that lived in their respective England. In 1860, John Phillips, processes, accumulated deposits,
depositional environments. By a geologist at Oxford University, and history. However, they
the 1850s, the geological column estimated that Earth is about had no scientific method with
of strata (also known as the 96 million years old. which to contradict Kelvin.
stratigraphic column) had been
more or less carved up into some But in 1862, such geological In the 1890s, the discovery
eight named systems of strata and calculations were scorned by the of naturally occurring radioactive
fossils, each of which represented eminent Scottish physicist William elements in some of Earth’s
a period of geological time. Thomson (Lord Kelvin) for being minerals and rocks provided the
unscientific. Kelvin was a strict key that would resolve the impasse
Geologists were impressed by empiricist and argued that he between Kelvin and the geologists,
the overall thickness of the strata, could use physics to determine since the rate at which atoms
estimated to be 16–70 miles an accurate age for Earth, which decay makes a reliable timer.
(25–112 km) thick. They had he thought was constrained by In 1903, Ernest Rutherford
the age of the Sun. Understanding predicted rates of radioactive
of Earth’s rocks, their melting decay and suggested that
points and conductivity, had vastly radioactivity might be used as
improved since Buffon’s day. Kelvin a “clock” to date minerals and
the rocks that contain them.
Lord Kelvin pronounced the world to
be 40 million years old in 1897, the year In 1905, Rutherford obtained
in which radioactivity was discovered. the very first radiometric dates
He did not know that radioactive decay of formation for a mineral from
in Earth’s crust provides heat that Glastonbury, Connecticut: 497–500
greatly slows the rate of cooling. million years. He warned that these
were minimum dates. In 1907,
American radiochemist Bertram
Boltwood improved on Rutherford’s
technique to produce the first
radiometric dates of minerals in
rocks with a known geological
context. These included a
2.2-billion-year-old rock from
Sri Lanka, whose age increased
previous estimates by an order

EXPANDING HORIZONS 101

An uncomformity is a buried surface separating two rock
strata of different ages. This diagram shows an angular
unconformity, similar to those discovered by James Hutton
on the east coast of Scotland. Here, layers of rock strata have
been tilted by volcanic activity or movements in Earth’s
crust, producing an angular discordance with overlying,
younger layers.

Angular James Hutton
discordance
Born in 1726 to a respected
Older, tilted merchant in Edinburgh,
rock strata Scotland, James Hutton
studied humanities at
of magnitude. By 1946, British radiometric age of 4.56 billion Edinburgh University. He
geologist Arthur Holmes had made years for granite and basalt igneous became interested in
some isotope measurements from rocks in Earth’s crust, he concluded chemistry and then medicine,
lead-bearing rocks from Greenland, that the similarity of dates was but did not practice as a
which gave an age of 3.015 billion indicative of the age of Earth’s doctor. Instead, he studied
years. This was one of the first formation. By 1956, he had made the new agrarian techniques
reliable minimum ages for Earth. further measurements, which being used in East Anglia,
Holmes went on to estimate the increased his confidence in the England, where his exposure
age of the uranium from which the accuracy of the date of 4.55 billion to soils and the rocks they
lead was derived, obtaining a date years. This remains the figure were derived from led to an
of 4.46 billion years, but he thought accepted by scientists today. ■ interest in geology. This took
that must be the age of the gas him on field expeditions all
cloud from which Earth formed. The past history of our over England and Scotland.
globe must be explained by
Finally, in 1953, American Returning to Edinburgh
geochemist Clair Patterson what can be seen to be in 1768, Hutton became
obtained the first generally happening now. acquainted with some of
accepted radiometric age of 4.55 James Hutton the major figures of the
billion years for Earth’s formation. Scottish Enlightenment,
There are no known minerals or including the engineer
rocks dating from Earth’s origin, James Watt and the moral
but many meteorites are thought philosopher Adam Smith.
to originate from the same event Over the next 20 years, Hutton
in the solar system. Patterson developed his famous theory
calculated the radiometric date for of Earth’s age and discussed it
lead minerals in the Canyon Diablo with his friends before finally
meteorite at 4.51 billion years. publishing a long outline in
Comparing it with the average 1788 and a much longer book
in 1795. He died in 1797.

Key work

1795 Theory of the Earth
with Proofs and Illustrations

102

THE ATTRACTION
OF MOUNTAINS

NEVIL MASKELYNE (1732–1811)

IN CONTEXT The gravitational mass The plumb line
of a mountain should will hang at an angle that
BRANCH attract a plumb bob.
Earth science and physics depends on the relative
density of the mountain
BEFORE
1687 Isaac Newton publishes and Earth.
the Principia, in which he
suggests experiments for Measuring the deviation
calculating Earth’s density. should allow calculation
of Earth’s mass.
1692 In an effort to explain
Earth’s magnetic field, I n the 17th century, Isaac However, Newton himself
Edmond Halley suggests that Newton had suggested dismissed the idea because
the planet consists of three methods for “weighing the he thought the deviation would
concentric hollow spheres. Earth”—or calculating Earth’s be too small to be measured
density. One of these involved with the instruments of the day.
1738 Pierre Bouguer attempts measuring the angle of a plumb
Newton’s experiment, without line on each side of a mountain to In 1738, Pierre Bouguer, a
success, on Chimborazo, a find out how far the gravitational French astronomer, attempted
volcano in Ecuador. attraction of the mountain pulled the experiment on the slopes of
it from the vertical. This deviation Chimborazo in Ecuador. Weather
AFTER could be measured by comparing and altitude caused problems,
1798 Henry Cavendish the plumb line to a vertical however, and Bouguer did not think
uses a different method to calculated using astronomical his measurements were accurate.
calculate the density of methods. If the density and
Earth, and finds it to be volume of the mountain could be In 1772, Nevil Maskelyne
340 lb/ft3 (5,448 kg/m3). ascertained, then, by extension, proposed to the Royal Society in
so could the density of Earth. London that the experiment could
1854 George Airy figures be conducted in Britain. The
out Earth’s density using Society agreed, and sent a surveyor
pendulums in a mine.

EXPANDING HORIZONS 103

See also: Isaac Newton 62–69 ■ Henry Cavendish 78–79 ■ John Michell 88–89

to select an appropriate mountain. Schiehallion was chosen as the site …the mean density of the
He chose Schiehallion in Scotland, for the experiment because it was earth is at least double of that
and Maskelyne spent nearly four symmetrically shaped and isolated at the surface…the density of
months making observations from (and therefore less affected by the the internal parts of the earth
both sides of the mountain. gravitational pull of other mountains).
is much greater than near
The density of rocks arc (just over 0.003 degrees). the surface.
The orientation of the plumb line Maskelyne used a survey of the
compared to the stars should have shape of the mountain and a Nevil Maskelyne
been different at the two stations measurement of the density of its
even without any gravitational rocks to figure out the mass of that said Earth was hollow. It also
effects, because of the difference Schiehallion. He was assuming allowed the mass of Earth to be
in latitude. However, even when that the whole Earth had the same extrapolated from its volume and
this was accounted for, there was density as Schiehallion, but the average density. Maskelyne’s value
still a difference of 11.6 seconds of deviation of the plumb lines for the overall density of Earth was
showed a measured value of less 280 lb/ft3 (4,500 kg/m3). Compared
than half of what he was expecting. with today’s accepted value of
Maskelyne realized that the density 344 lb/ft3 (5,515 kg/m3), he had
assumption was not correct—the figured out the density of Earth
density of Earth was clearly much with an error of less than 20 percent,
greater than that of its surface and in the process had proved
rocks, probably, he reasoned, due Newton's law of gravitation. ■
to the planet having a metallic
core. The actual observed angle
was used to figure out that the
overall density of Earth is about
double that of Schiehallion’s rocks.

This result disproved one
theory of the time, advocated by
English astronomer Edmond Halley,

Nevil Maskelyne Born in London in 1732, Nevil much time trying to solve the
Maskelyne became interested problem of measuring longitude
in astronomy at school. After while at sea—a major issue of
graduating from Cambridge the day. His method involved
University and being ordained a carefully measuring the
priest, he became a member of distance between the Moon
the Royal Society in 1758, and and a given star, and consulting
was the Astronomer Royal from published tables.
1765 until his death.
Key works
In 1761, the Royal Society sent
Maskelyne to the Atlantic island of 1764 Astronomical Observations
St. Helena to observe the transit Made at the Island of St Helena
of Venus. Measurements taken 1775 An Account of
as the planet passed across the Observations Made on
Sun’s disk allowed astronomers to the Mountain Schehallien
calculate the distance between for Finding its Attraction
Earth and the Sun. He also spent

104

THE MYSTERY OF NATURE
IN THE STRUCTURE AND
FERTILIZATION OF FLOWERS

CHRISTIAN SPRENGEL (1750–1816)

IN CONTEXT I n the mid-18th century, Swedish and female parts, and that in these,
botanist Carl Linnaeus realized the parts mature at different times,
BRANCH that flower parts parallel the preventing self-fertilization.
Biology reproductive organs of animals.
Forty years later, a German Published in 1793, Sprengel’s
BEFORE botanist called Christian Sprengel work was largely underappreciated
1694 German botanist figured out how insects played a during his lifetime. However,
Rudolph Camerarius shows major role in the pollination, and it was finally given due credit
that flowers carry a plant’s so fertilization, of flowering plants. when Charles Darwin used it as
reproductive parts. a springboard for his own studies
Mutual benefit on the coevolution of flowering
1753 Carl Linnaeus publishes In the summer of 1787, Sprengel plants and the particular species
Species Plantarum, devising a noticed insects visiting open of insects that pollinate them and
classification system guided flowers to feed on the nectar inside. ensure cross-fertilization—to their
by flower structure. He began to wonder whether the mutual benefit. ■
nectar was being “advertised”
1760s Josef Gottlieb Kölreuter, by the petals’ special color and A honeybee lands on the sexual parts
a German botanist, proves pattern, and deduced that the displayed at the center of these brightly
that pollen grains are needed insects were being enticed onto colored petals. Honeybees account for
to fertilize a flower. the flowers so that pollen from the 80 percent of all insect pollination and
stamen (male part) of one flower pollinate a third of all food crops.
AFTER stuck to the insect and was carried
1831 Scottish botanist Robert to the pistil (female part) of another
Brown describes how pollen flower. The insect’s reward was a
grains germinate on a flower’s drink of energy-rich nectar.
stigma (female part).
Sprengel discovered that some
1862 Charles Darwin flowering plants, if they lack color
publishes Fertilisation of and scent, rely on wind to disperse
Orchids, a detailed study their pollen. He also observed that
of the relationship between many flowers contain both male
flowers and pollinating insects.
See also: Carl Linnaeus 74–75 ■ Charles Darwin 142–49 ■
Gregor Mendel 166–71 ■ Thomas Hunt Morgan 224–25

EXPANDING HORIZONS 105

ELEMENTS
ALWAYS COMBINE
THE SAME WAY

JOSEPH PROUST (1754–1836)

IN CONTEXT T he Law of Definite Iron, like many other metals,
Proportions, published is subject to the law of nature
BRANCH by French chemist Joseph
Chemistry Proust in 1794, shows that no which presides at every
matter how elements combine, the true combination, that
BEFORE proportions of each element in a is to say, that it unites
c.400 BCE The Greek thinker compound are always precisely the
Democritus proposes that the same. This theory was one of the with two constant
world is ultimately made of tiny fundamental ideas about elements proportions of oxygen.
indivisible particles—atoms. that emerged at this period to form
the basis of modern chemistry. Joseph Proust
1759 English chemist Robert
Dossie argues that substances In making his discovery, Proust John Dalton’s new atomic theory of
combine when they are in the was following a trend in French elements—that elements are each
right proportion, which he calls chemistry, pioneered by Antoine made of their own unique atoms. If
the “saturation proportion.” Lavoisier, which advocated careful a compound is always made from
measurement of weights, ratios, the same combination of atoms,
1787 Antoine Lavoisier and and percentages. Proust studied Proust’s argument that elements
Claude Louis Berthollet devise the percentages in which metals always combine in fixed
the modern system of naming combined with oxygen in metal proportions must be true. This is
chemical compounds. oxides. He concluded that when now accepted as one of the key
metal oxides formed, the proportion laws of chemistry. ■
AFTER of metal and oxygen was constant.
1805 John Dalton shows that If the same metal combined with
elements are made up of atoms oxygen in a different proportion, it
of a particular mass, which formed a different compound with
combine to make compounds. different properties.

1811 Italian chemist Amedeo Not everyone agreed with
Avogadro makes a distinction Proust, but in 1811, the Swedish
between atoms and the chemist Jöns Jakob Berzelius
molecules that are formed by realized that Proust’s theory fit
atoms to make compounds.
See also: Henry Cavendish 78–79 ■ Antoine Lavoisier 84 ■ John Dalton 112–13 ■
Jöns Jakob Berzelius 119 ■ Dmitri Mendeleev 174–79

A CENT
OF PRO

1800–1900

URY
GRESS

108 INTRODUCTION

Astronomer William In the cliffs of Lyme Michael Faraday Christian Doppler
Herschel discovers Regis, Mary Anning finds discovers the principle explains why binary
infrared radiation.
the skeleton of the first behind the stars are colored.
known ichthyosaur. electric motor.

1800 1811 1821 1842
1803 1820 1837 1845

John Dalton introduces the Hans Christian Ørsted Louis Agassiz German explorer
idea of atomic weights. discovers that when a describes Alexander von
current is switched on, an ice age. Humboldt introduces
the idea of ecology.
a nearby compass
needle flickers.

T he invention of the electric William Herschel used a prism to of binary stars using the idea that
battery in 1799 opened separate the various colors of light is a wave with a spectrum
up whole new fields of sunlight to study their of various frequencies, laying out
scientific research. In Denmark, temperatures; he found that his the phenomenon now known as the
Hans Christian Ørsted accidentally thermometer showed a higher Doppler effect. Meanwhile, in Paris,
discovered a connection between temperature beyond the red end of French physicists Hippolyte Fizeau
electricity and magnetism. At the visible spectrum. Herschel had and Léon Foucault measured the
London’s Royal Institution, stumbled upon infrared radiation, speed of light, and showed that it
Michael Faraday imagined the and ultraviolet radiation was travels more slowly through water
shapes of magnetic fields, and discovered the following year— than through air.
invented the world’s first electric proving that there was more to the
motor. In Scotland, James Clerk spectrum than visible light. In a Chemical changes
Maxwell picked up Faraday’s similar accidental way, Wilhelm British meteorologist John Dalton
ideas and figured out the complex Röntgen later discovered X-rays in tentatively suggested that atomic
mathematics of electromagnetism. his laboratory in Germany. British weights might be a useful concept
physician Thomas Young devised for chemists and ventured to
Seeing the invisible a clever double-slit experiment to estimate a few of them. Fifteen
Invisible forms of electromagnetic determine whether light is really a years later, Swedish chemist Jöns
waves were discovered before wave or a particle. His discovery Jakob Berzelius drew up a much
they were understood or the laws of wavelike interference appeared more complete list of atomic
governing their behavior were to settle the argument. In Prague, weights. His student, the German
figured out. Working in Bath, Austrian physicist Christian chemist Friedrich Wöhler, turned
Britain, German astronomer Doppler explained the color an inorganic salt into an organic

A CENTURY OF PROGRESS 109

Charles Darwin August Kekulé Dmitri Mendeleev lays Wilhelm Röntgen
outlines his theory of describes the out the periodic table discovers X-rays.
evolution in On the chemical structure
of the benzene of the elements.
Origin of Species by
Means of Natural molecule.
Selection.

1859 1865 1869 1895
1859 1866 1873 1898

Louis Pasteur disproves Gregor Mendel James Clerk Maxwell Marie Curie
spontaneous publishes his work on publishes his laws of isolates radioactive
the genetics of peas. electromagnetism.
generation of life. polonium.

compound, and so disproved the Mary Anning documented a series and Charles Darwin both hit on
idea that life chemistry operated of fossils of extinct creatures she the idea of evolution by means
according to separate rules. In had dug out of the cliffs. Soon of natural selection. T. H. Huxley
Paris, Louis Pasteur further showed afterward, Richard Owen coined demonstrated that birds may well
that life cannot be generated the word “dinosaurs” to describe have evolved from dinosaurs, and
spontaneously. Inspiration for new the “terrible lizards” that had once the evidence to support evolution
ideas came from various quarters. roamed the planet. Swiss geologist mounted. Meanwhile, a German-
The structure of the benzene Louis Agassiz suggested that large speaking Silesian friar named
molecule came to German chemist parts of Earth had once been Gregor Mendel sorted out the
August Kekulé as he drifted off covered with ice, further expanding basic laws of genetics by studying
to sleep, while Russian chemist the idea that Earth has experienced thousands of pea plants. Mendel’s
Dmitri Mendeleev used a pack of very different conditions through its work would be neglected for some
cards to crack the problem of the history. Alexander von Humboldt decades, but its rediscovery would
periodic table of the elements. used cross-disciplinary insights to provide the genetic mechanism
Marie (Skłodowska) Curie isolated uncover the connections in nature for natural selection.
polonium and radium, and became and established the study of
the only person to win Nobel prizes ecology. In France, Jean-Baptiste In 1900, British physicist
in both Chemistry and Physics. Lamarck outlined a theory of Lord Kelvin is alleged to have
evolution, mistakenly believing said “There is nothing new to be
Clues from the past that the passing on of acquired discovered in physics now. All
The century saw nothing short of a characteristics was its driving that remains is more and more
revolution in the understanding of force. Then, in the 1850s, British precise measurement.” Little can
life. On the south coast of England, naturalists Alfred Russel Wallace he have suspected what shocks
were just around the corner. ■

110

THE EXPERIMENTS
MAY BE REPEATED
WITH GREAT EASE
WHENTHESUN SHINES

THOMAS YOUNG (1773–1829)

IN CONTEXT If light is made of particles A t the turn of the 19th
that travel in straight century, scientific opinion
BRANCH was divided over the
Physics lines, then this can be proved question of the nature of light. Isaac
in a simple experiment… Newton had argued that a beam
BEFORE of light is made of countless,
1678 Christiaan Huygens first Shine a light through tiny, fast-moving “corpuscles”
proposes that light travels as two adjacent slits onto a (particles). If light consists of these
waves. He publishes his screen. Two pools of light bulletlike corpuscles, he said, this
Treatise on Light in 1690. would explain why light travels in
should be seen on straight lines and casts shadows.
1704 In his book Opticks, the screen.
Isaac Newton suggests that But Newton’s corpuscles did not
light comprises streams of But instead, it creates explain why light refracts (bends
particles, or “corpuscles.” interfering patterns of when it enters glass) or splits into
light and dark, just as water the colors of the rainbow—also
AFTER waves would if water flowed an effect of refraction. Christiaan
1905 Albert Einstein argues Huygens had argued that light
that light must be thought through two slits. comprises not particles, but waves.
of as particles, later called If light travels as waves, Huygens
photons, as well as waves. Light must travel as waves. said, it is easy to explain these
effects. However, Newton’s stature
1916 US physicist Robert was such that most scientists
Andrews Millikan proves backed the particle theory.
Einstein correct through
experiment. Then, in 1801, British physician
and physicist Thomas Young hit on
1961 Claus Jönsson repeats a design for a simple yet ingenious
Young’s double-slit experiment experiment that would, he believed,
with electrons, and shows settle the question one way or the
that, like light, they can other. The idea began when Young
behave as waves as well was looking at the patterns of
as particles. light made by a candle shining
through a mist of fine water
droplets. The pattern showed
colored rings around a bright

A CENTURY OF PROGRESS 111

See also: Christiaan Huygens 50–51 ■ Isaac Newton 62–69 ■
Léon Foucault 136–37 ■ Albert Einstein 214–21

center, and Young wondered if
the rings might be caused by
interacting waves of light.

The double-slit experiment Scientific investigations are Thomas Young
Young made two slits in a piece of a sort of warfare carried on
cardboard and shone a beam of against all one’s contemporaries The eldest of 10 children
light onto them. On a paper screen raised by Quaker parents in
placed behind the slits, the light and predecessors. Somerset, England, Thomas
created a pattern that convinced Thomas Young Young’s brilliant mind made
Young that it was waves. If light him a child prodigy, and he
were streams of particles, as wavelength. For a century, Young’s was nicknamed the “Young
Newton said, there should simply double-slit experiment convinced Phenomenon.” At 13, he could
have been a strip of light directly scientists that light is a wave, not read five languages fluently—
beyond each slit. Instead, Young a particle. Then in 1905, Albert as an adult, he made the
saw alternating bright and dark Einstein showed that light also first modern translation of
bands, like a fuzzy bar code. He behaves as if it were a stream of Egyptian hieroglyphics.
argued that as light waves spread particles—it can behave like a
out beyond the slits, they interact. If wave and a particle. Such was the After medical training
two waves ripple up (peak) or down simplicity of Young’s experiment in Scotland, Young set up
(trough) at the same time, they make that, in 1961, German physicist as a physician in London
a wave twice as big (constructive Claus Jönsson used it to show that in 1799, but he was a true
interference)—creating the bright the subatomic particles electrons polymath who, in his spare
bands. If one wave ripples up as the produce similar interference, so that time, conducted inquiries
other ripples down, they cancel each they, too, must also be waves. ■ into everything from a
other out (destructive interference)— theory of musical tuning to
creating the dark bands. Young also linguistics. He is most famous,
showed that different colors of light however, for his work on light.
create different interference In addition to establishing
patterns. This demonstrated that the principle of interference
the color of light depends on its of light, he devised the first
modern scientific theory of
Here, light travels Light waves color vision, arguing that
through two slits in a piece we see colors as varying
of card, and reaches a Card with proportions of the three main
screen. The light waves two slits colors: blue, red, and green.
passing through the slits
interfere. Where peaks Destructive Key works
(yellow) intersect with interference
troughs (blue), there is 1804 Experiments and
destructive interference. Pattern of light intensity Calculations Relative to
Where peaks intersect Physical Optics
with peaks and troughs 1807 Course of Lectures on
with troughs, there is Natural Philosophy and the
constructive interference. Mechanical Arts

Constructive interference

Screen

112

ASCERTAINING THE
RELATIVE WEIGHTS OF
ULTIMATE PARTICLES

JOHN DALTON (1766–1844)

IN CONTEXT Elements combine These fixed ratios must
with each other to make depend on the relative
BRANCH weight of the atoms of
Chemistry compounds in simple
fixed ratios. each element.
BEFORE
c.400 BCE Democritus Tables of elements Therefore, the atomic
proposes that the world is should be based on weight of an element
made of indivisible particles. the weights of their can be calculated from
ultimate particles. the weight of each element
8th century CE Persian involved in a compound.
polymath Jabir ibn Hayyan (or
Geber) classifies elements into T oward the end of the The idea of atoms dates back to
metals and non-metals. 18th century, scientists ancient Greece, but it had always
had begun to realize that been assumed that all atoms were
1794 Joseph Proust shows that the world is made up of a range identical. Dalton’s breakthrough
compounds are always made of basic substances, or chemical was to understand that each
of elements combined in the elements. But no one was certain element is made from different
same proportions. what an element was. It was John atoms. He described the atoms that
Dalton, an English meteorologist, made up the elements then known—
AFTER who, through his study of weather, including hydrogen, oxygen, and
1811 Amedeo Avogadro shows saw that each element is made nitrogen—as “solid, massy, hard,
that equal volumes of different wholly of its own unique, identical impenetrable, moveable particles.”
gases contain equal numbers atoms, and it is this special atom
of molecules. that distinguishes and defines Dalton’s ideas originated in
an element. In developing the his study of the way in which
1869 Dmitri Mendeleev draws atomic theory of elements, Dalton air pressure affected how much
up a periodic table, displaying established the basis of chemistry. water could be absorbed by air.
elements by atomic weight. He became convinced that air is

1897 Through his discovery
of the electron, J. J. Thomson
shows that atoms are not the
smallest possible particle.

A CENTURY OF PROGRESS 113

See also: Joseph Proust 105 ■ Dmitri Mendeleev 174–79

An inquiry into the by differences in their weights. Dalton’s work had put scientists on
relative weight of the ultimate He saw that the atoms, or “ultimate the right track, and within a decade
particles of bodies is a subject, particles,” of two or more elements Italian physicist Amedeo Avogadro
as far as I know, entirely new. combined to make compounds in had devised a system of molecular
very simple ratios, so he could proportions to calculate atomic
John Dalton figure out the weight of each atom weights correctly. Yet the basic
by the weight of each element idea of Dalton’s theory—that each
a mixture of different gases. As he involved in a compound. Very element has its own unique-sized
experimented, he observed that a quickly, he figured out the atomic atoms—has proved to be true. ■
given quantity of pure oxygen will weight of each element then known.
take up less water vapor than the
same amount of pure nitrogen, Hydrogen, Dalton realized, was
and he jumped to the remarkable the lightest gas, so he assigned it
conclusion that this is because an atomic weight of 1. Because
oxygen atoms are bigger and of the weight of oxygen that
heavier than nitrogen atoms. combined with hydrogen in water,
he assigned oxygen an atomic
Weighty matters weight of 7. However, there was a
In a flash of insight, Dalton flaw in Dalton’s method, because
realized that atoms of different he did not realize that atoms of the
elements could be distinguished same element can combine. He
always assumed that a compound
of atoms—a molecule—had only
one atom of each element. But

Dalton’s table shows symbols and
atomic weights of different elements.
Dalton was drawn to atomic theory
through meteorology, when he asked
himself why air and water particles
could mix.

John Dalton Born into a Quaker family in papers for the Society, including
England’s Lake District in 1766, those about his atomic theory.
John Dalton made regular The atomic theory was quickly
observations of the weather from accepted, and Dalton became a
the age of 15. These provided celebrity in his own lifetime—
many key insights, such as that more than 40,000 people
atmospheric moisture turns to rain attended his funeral in
when the air cools. In addition to Manchester in 1844.
his meteorological studies, Dalton
became fascinated by a condition Key works
he and his brother shared: color
blindness. His scientific paper on 1805 Experimental Enquiry into
the subject gained him admission the Proportion of the Several
to the Manchester Literary and Gases or Elastic Fluids,
Philosophical Society, of which Constituting the Atmosphere
he was elected president in 1817. 1808–27 New System of
He wrote hundreds of scientific Chemical Philosophy

114

THE CHEMICAL
EFFECTS PRODUCED
BY ELECTRICITY

HUMPHRY DAVY (1778–1829)

IN CONTEXT I n 1800, Alessandro Volta Davy used apparatus similar to
invented the “voltaic pile”— this in his lectures at London’s Royal
BRANCH the world’s first battery, and Institution to show how electrolysis
Chemistry soon many other scientists began splits water into its two elements,
to experiment with batteries. hydrogen and oxygen.
BEFORE
1735 Swedish chemist English chemist Humphry Davy the same way and produced the
Georges Brandt discovers realized that the battery’s electricity metal sodium. In 1808, he used
cobalt, the first of many new is produced by a chemical reaction. electrolysis to discover four more
metallic elements to be found Electric charge flows as the pile’s metallic elements—calcium, barium,
over the next 100 years. two different metals (the electrodes) strontium, and magnesium—and
react via the brine-soaked paper the metalloid boron. Like electrolysis,
1772 Italian physician Luigi between them. In 1807, Davy found their commercial use would prove
Galvani notices the effect that he could use the electric highly valuable. ■
of electricity on a frog and charge from a pile to split chemical
believes electricity is biological. compounds, discovering new
elements, and pioneering a process
1799 Alessandro Volta shows that was later called electrolysis.
that touching metals produce
electricity, and creates the New metals
first battery. Davy inserted two electrodes into
dry potassium hydroxide (potash),
AFTER which he moistened by exposing it
1834 Davy’s former assistant to the damp air in his laboratory so
Michael Faraday publishes the that it would conduct electricity. To
laws of electrolysis. his delight, metallic globules began
to form on the negatively charged
1869 Dmitri Mendeleev electrode. The globules were a new
arranges the known elements element: the metal potassium. A
into a periodic table, creating a few weeks later, he electrolyzed
group for the soft alkali metals sodium hydroxide (caustic soda) in
that Davy had been the first
to identify in 1807. See also: Alessandro Volta 90–95 ■ Jöns Jakob Berzelius 119 ■
Hans Christian Ørsted 120 ■ Michael Faraday 121 ■ Dmitri Mendeleev 174–79

A CENTURY OF PROGRESS 115

MAPPING THE
ROCKS OF
A NATION

WILLIAM SMITH (1769–1839)

IN CONTEXT I n the mid to late 18th century, France, who mapped the geology
the need to find fuels and ores of the Paris Basin in 1811, and
BRANCH to power Europe’s Industrial William Smith in Britain.
Geology Revolution spurred a growing
interest in producing geological First national map
BEFORE maps. German mineralogists Smith was a self-taught engineer
1669 Nicholas Steno publishes Johann Lehmann and Georg and surveyor who produced the first
the principles of stratigraphy Füchsel produced detailed aerial nationwide geological map in 1815,
that will guide geologists’ views showing topography and showing England, Wales, and part
understanding of rock strata. rock strata. Many subsequent of Scotland. By amassing samples
geological maps did little more than from mines, quarries, cliffs, canals,
1760s In Germany, geologists show the surface distribution of and road and railroad cuttings,
Johann Lehmann and Georg different rock types—until the Smith established the succession
Füchsel make some of the first pioneering work of Georges Cuvier of rock strata, using Steno’s
measured sections and maps and Alexandre Brongniart in principles of stratigraphy and
of geological strata. identifying each stratum by its
Organized fossils are to characteristic fossils. He also drew
1813 English geologist Robert the naturalist as coins vertical sections of the succession
Bakewell makes the first of strata and the geological
geognostic map of rock types to the antiquary. structures into which they had
in England and Wales. William Smith been formed by earth movements.

AFTER Over the next few decades, the
1835 The Geological Survey first national geological surveys
of Great Britain is founded to were established, and they set
conduct systematic geological about methodically mapping their
mapping of the country. entire countries. The correlation of
strata of similar age across national
1878 The first International boundaries was achieved by
Geological Congress is held in international agreement in the
Paris. Congresses have been latter part of the 19th century. ■
held every three to five years
ever since. See also: Nicholas Steno 55 ■ James Hutton 96–101 ■ Mary Anning 116–17 ■
Louis Agassiz 128–29

116

SHE KNOWS TO
WHAT TRIBE THE
BONES BELONG

MARY ANNING (1799–1847)

IN CONTEXT Fossils are the preserved Fossils have been
remains of plants found of large animals
BRANCH and animals. no longer around today.
Paleontology
In the past, very different animals lived on Earth.
BEFORE
11th century Persian scholar B y the end of the 18th did they fit into the classification
Avicenna (Ibn Sina) suggests century, it was generally systems, and when had they
that rocks can be formed from accepted that fossils become extinct? Within the Judeo-
petrified fluids, leading to the were the remains of once living Christian culture of the Western
formation of fossils. organisms that had been petrified world, it was generally thought that
as the sediment around them a benevolent God would not have
1753 Carl Linnaeus includes hardened into rock. Both fossils allowed any of his creations to
fossils in his system of and living organisms had been die out.
biological classification. classified for the first time into
a hierarchy of species, genera, Monsters from the abyss
AFTER and families by naturalists such Some of the first of these large
1830 British artist Henry De as the Swedish taxonomist Carl and distinctive fossil remains were
la Beche paints one of the first Linnaeus. However, fossil remains found by the Anning family of fossil
paleo-reconstructions of a were still seen in isolation collectors around Lyme Regis on
scene from “deep time.” from their environmental and the coast of southern England.
biological context. Here, Jurassic-period limestone and
1854 Richard Owen and shale strata outcrop in the cliffs,
Benjamin Waterhouse In the early 19th century, the where they are eroded by the sea to
Hawkins make the first discovery of large fossilized bones reveal abundant remains of ancient
life-size reconstructions of unlike those of any living animal marine organisms. In 1811, Joseph
extinct plants and animals. raised many new questions. Where

Early 20th century The
development of radiometric
dating techniques allows
scientists to date fossils
according to the rock strata
in which they are found.

A CENTURY OF PROGRESS 117

See also: Carl Linnaeus 74–75 ■ Charles Darwin 142–49 ■
Thomas Henry Huxley 172–73

Anning found a 4 ft- (1.2 m-) long observed in 1824, Mary Anning Mary Anning
skull with a curiously elongated was “so thoroughly acquainted
toothed beak. His sister Mary with the science that the moment Several biographies and
found the rest of the skeleton, she finds any bones she knows to novels have been written
which they sold for about $37 (£23). what tribe they belong.” She about the life of Mary Anning,
Exhibited in London, this was the became an authority on many a self-taught fossil collector.
first entire skeleton of an extinct kinds of fossils, especially She was one of two surviving
“monster of the abyss” and coprolites—fossilized dung. children out of 10 born into an
attracted a great deal of popular impoverished Dorset family of
attention. It was identified as an The picture of life in ancient religious dissenters who lived
extinct marine reptile and named Dorset revealed by Anning’s fossils in the coastal village of Lyme
an ichthyosaur, meaning “fish-lizard.” was one of a tropical coast where Regis. The family eked out a
a wide variety of now-extinct precarious living collecting
The Anning family went on to animals thrived. In 1854, Anning’s fossils for sale to the growing
find more ichthyosaurs and the fossils provided models for the first numbers of tourists. However,
first complete specimen of another life-size reconstruction of an it was Mary who found and
marine reptile, the plesiosaur, ichthyosaur, made for London’s sold their most significant
in addition to the first British Crystal Palace park by the sculptor finds—fossils of Jurassic
specimen of a flying reptile, new Benjamin Waterhouse Hawkins and reptiles that lived 201–145
fossil fish, and shellfish. Among the the paleontologist Richard Owen. It million years ago.
fish they found were cephalopods was Owen who coined the word
known as belemnites, some with “dinosaur,” but Anning who had Due to a combination of
the ink-sac preserved. The family, provided the first glimpse of the her gender, humble social
and especially Mary, had a talent richness of Jurassic life. ■ standing, and religious
for fossil hunting. Although poor, unorthodoxy, Anning received
Mary was literate and taught In 1830, Henry De la Beche little formal recognition of her
herself geology and anatomy, which painted this reconstruction of life in work in her lifetime, and she
made her a far more effective fossil the Jurassic seas around Dorset based noted in a letter, “The world
hunter. As Lady Harriet Sylvester on Anning’s fossil discoveries. has used me unkindly, I fear
it has made me suspicious of
everyone.” However, she was
widely known in geological
circles and various scientists
sought out her expertise.
When her health failed,
Anning was provided with a
small annual pension of about
$40 (£25) in recognition of her
contribution to science. She
died of breast cancer at 47.

118

THE INHERITANCE
OF ACOUIRED
CHARACTERISTICS

JEAN-BAPTISTE LAMARCK (1744–1829)

IN CONTEXT I n 1809, French naturalist What nature does in the
Jean-Baptiste Lamarck course of long periods we do
BRANCH introduced the first major theory every day when we suddenly
Biology that life on Earth has evolved over change the environment in
time. The impetus to his theory which some species of living
BEFORE was the discovery of fossils of
c.1495 Leonardo da Vinci creatures unlike any alive today. plant is situated.
suggests in his notebook that In 1796, French naturalist Georges Jean-Baptiste Lamarck
fossils are relics of ancient life. Cuvier had shown that fossilized
elephant-like bones were markedly Lamarck believed characteristics
1796 Georges Cuvier proves different in anatomy from the bones were acquired during a creature’s
that fossil bones belong to of modern elephants, and must life and passed on. Later, Darwin
extinct mastodons. come from extinct creatures now showed that changes occur because
called mammoths and mastodons. mutations at conception survive
1799 William Smith shows the to be passed on through natural
succession of fossils in rock Cuvier explained the vanished selection, and the idea of “acquired
strata of different ages. creatures of the past as victims of characteristics” was ridiculed. But
catastrophes. Lamarck challenged recently, scientists have argued that
AFTER this idea, and argued that life the environment—chemicals, light,
1858 Charles Darwin had “transmutated,” or evolved, temperature, and food—can in fact
introduces his theory of gradually and continuously through alter genes and their expression. ■
evolution by natural selection. time, developing from the simplest
life forms to the most complex. A
1942 The “modern synthesis” change in the environment, he
reconciles Gregor Mendel’s suggested, could spur a change in
genetics with Darwin’s natural the characteristics of an organism.
selection, paleontology, and Those changes could then be
ecology in trying to explain inherited through reproduction.
how new species arise. Characteristics that were useful
developed further; those that were
2005 Eva Jablonka and Marion not useful might disappear.
Lamb claim that nongenetic,
environmental, and behavioral See also: William Smith 115 ■ Mary Anning 116–17 ■ Charles Darwin 142–49 ■
changes can affect evolution. Gregor Mendel 166–71 ■ Thomas Hunt Morgan 224–25 ■ Michael Syvanen 318–19

A CENTURY OF PROGRESS 119

EVERY CHEMICAL
COMPOUND HAS
TWO PARTS

JÖNS JAKOB BERZELIUS (1779–1848)

IN CONTEXT T he leading light of a In 1803, Berzelius had teamed
generation of chemists up with a mine owner to make a
BRANCH inspired by Alessandro voltaic pile and see how electricity
Chemistry Volta’s creation of the battery, splits salts. Alkali metals and
Sweden’s Jöns Jakob Berzelius alkaline earths migrated to the
BEFORE conducted a series of experiments pile’s negative pole, while oxygen,
1704 Isaac Newton suggests looking at the effect of electricity on acids, and oxidized substances
that atoms are bonded by chemicals. He developed a theory migrated to the positive pole.
some force. called electrochemical dualism, He concluded that salt compounds
published in 1819, which proposed combine a basic oxide, which is
1800 Alessandro Volta shows that compounds are created by the positively charged, and an acidic
that placing two different coming together of elements with oxide, which is negatively charged.
metals next to each other can opposite electrical charges.
produce electricity, and so Berzelius developed his
creates the first battery. The habit of an opinion dualistic theory to suggest that
often leads to the complete compounds are bonded by the
1807 Humphry Davy discovers conviction of its truth, and attraction of opposite electrical
sodium and other metal charges between their constituent
elements by splitting salts makes us incapable of parts. Though later shown to be
with electrolysis. accepting the proofs against it. incorrect, the theory triggered
further research into chemical
AFTER Jöns Jakob Berzelius bonds. In 1916, it was found that
1857–58 August Kekulé electrical bonding occurs as “ionic”
and others develop the idea bonding, in which atoms lose or
of valency—the number of gain electrons to become mutually
bonds an atom can form. attractive charged atoms, or ions.
In fact, this is just one of several
1916 US chemist Gilbert ways in which the atoms in a
Lewis proposes the idea of compound bind—another is the
the covalent bond in which “covalent” bond, in which electrons
electrons are shared, while are shared between atoms. ■
German physicist Walther
Kossel suggests the idea See also: Isaac Newton 62–69 ■ Alessandro Volta 90–95 ■ Joseph Proust 105 ■
of ionic bonds. Humphry Davy 114 ■ August Kekulé 160–65 ■ Linus Pauling 254–59

120

THE ELECTRIC
CONFLICT IS NOT
RESTRICTED TO THE
CONDUCTING WIRE

HANS CHRISTIAN ØRSTED (1777–1851)

IN CONTEXT T he quest to discover an idea that there is a unity to nature,
underlying unity to all Ørsted now investigated the
BRANCH forces and matter is as old possibility in earnest.
Physics as science itself, but the first big
break came in 1820, when the Chance discovery
BEFORE Danish philosopher Hans Christian Lecturing at the University of
1600 William Gilbert conducts Ørsted found a link between Copenhagen, Ørsted wanted to
the first scientific experiments magnetism and electricity. The link show his students how the electric
on electricity and magnetism. had been suggested to him by the current from a voltaic pile (the
German chemist and physicist battery invented by Alessandro
1800 Alessandro Volta creates Johann Wilhelm Ritter, whom he Volta in 1800) can heat up a wire
the first electric battery. had met in 1801. Already influenced and make it glow. He noticed that a
by the philosopher Immanuel Kant’s compass needle standing near the
AFTER wire moved every time the current
1820 André-Marie Ampère It appears that the electric was switched on. This was the first
develops a mathematical conflict is not restricted to the proof of a link between electricity
theory of electromagnetism. and magnetism. Further study
conducting wire, but that it convinced him that the current
1821 Michael Faraday is able has a rather extended sphere produced a circular magnetic field
to show electromagnetic as it flowed through the wire.
rotation in action, by creating of activity around it.
the first electric motor. Hans Christian Ørsted Ørsted’s discovery rapidly
prompted scientists across Europe
1831 Faraday and US scientist to investigate electromagnetism.
Joseph Henry independently Later that year, French physicist
discover electromagnetic André-Marie Ampère formulated
induction; Faraday uses it in a mathematical theory for the
the first generator to convert new phenomenon and, in 1821,
motion into electricity. Michael Faraday demonstrated that
electromagnetic force could convert
1864 James Clerk Maxwell electrical into mechanical energy. ■
formulates a set of equations
to describe electromagnetic See also: William Gilbert 44 ■ Alessandro Volta 90–95 ■ Michael Faraday 121 ■
waves—including light waves. James Clerk Maxwell 180–85

A CENTURY OF PROGRESS 121

ONE DAY, SIR,
YOU MAY TAX IT

MICHAEL FARADAY (1791–1867)

IN CONTEXT British scientist Michael In Faraday’s apparatus for showing
Faraday’s discovery of electromagnetic induction, a current
BRANCH the principles of both the flows through the small magnetic coil,
Physics electric motor and the electric which is moved in and out of the large
generator paved the way for the coil, inducing a current in it.
BEFORE electrical revolution that would
1800 Alessandro Volta invents transform the modern world, Generating electricity
the first electric battery. bringing everything from lightbulbs Ten years later, Faraday made an
to telecommunications. Faraday even more important discovery—
1820 Hans Christian Ørsted himself foresaw the value of that a moving magnetic field can
discovers that electricity his discoveries—and the tax create or “induce” a current of
creates a magnetic field. revenues they could generate electricity. This discovery—which
for government. was also made independently by
1820 André-Marie Ampère the US physicist Joseph Henry
formulates a mathematical In 1821, a few months after around the same time—is the
theory of electromagnetism. hearing of Hans Christian Ørsted’s basis for generating all electricity.
discovery of the link between Electromagnetic induction converts
AFTER electricity and magnetism, Faraday the kinetic energy in a spinning
1830 Joseph Henry creates the demonstrated how a magnet will turbine into electrical current. ■
first powerful electromagnet. move around an electric wire, and
an electric wire will move around a
1845 Faraday demonstrates magnet. The electric wire produces
the link between light and a circular magnetic field around it,
electromagnetism. which generates a tangential force
on the magnet, producing circular
1878 Designed by Sigmund motion. This is the principle behind
Schuckert, the first steam- the electric motor. A spinning
driven power station generates motion is set up by alternating the
electricity for the Linderhof direction of the current, which
Palace in Bavaria, Germany. alternates the direction of the
magnetic field in the wire.
1882 US inventor Thomas
Edison builds a power station See also: Alessandro Volta 90–95 ■ Hans Christian Ørsted 120 ■
to power electric lighting in James Clerk Maxwell 180–85
Manhattan, New York City.

122

HEAT PENETRATES
EVERY SUBSTANCE
IN THE UNIVERSE

JOSEPH FOURIER (1777–1831)

IN CONTEXT Heat penetrates T oday, one of the most
every substance fundamental laws of
BRANCH in the universe. physics is that energy
Physics is neither created nor destroyed,
There is a temperature but only changes from one form to
BEFORE gradient between warmer another or moves from one place
1761 Joseph Black discovers to another. French mathematician
latent heat—the heat taken up places and cooler places. Joseph Fourier was a pioneer in the
by ice to melt and water to boil study of heat and how heat moves
without changing temperature. Heat is transferred across from warm places to cool places.
He also studies specific heat— the temperature gradient in
required by substances to Fourier was interested in both
raise their temperature by a a wavelike movement. how heat diffused through solids by
certain amount. conduction and how things cooled
Mathematically, a series down by losing heat. His compatriot
1783 Antoine Lavoisier and of sine and cosine Jean-Baptiste Biot had imagined
Pierre-Simon Laplace measure the spread of heat as “action at a
latent heat and specific heat. functions can be used to distance,” in which it spreads by
represent the movement. jumping from warm places to cool.
AFTER Biot represented the heat flow in
1824 By developing the a solid as a series of slices, which
first theory of heat engines, allowed it to be studied with
which turn heat energy into conventional equations showing
mechanical energy, Nicolas the heat jumping from one slice
Sadi Carnot provides the to the next.
foundations for the theory
of thermodynamics. Temperature gradients
Fourier looked at heat flow in an
1834 Émile Clapeyron shows entirely different way. He focused
that energy must always on temperature gradients—
become more diffuse, continuous gradations between
formulating the second law warm and cool places. These could
of thermodynamics. not be quantified with conventional
equations, so he devised new
mathematical techniques.

A CENTURY OF PROGRESS 123

See also: Isaac Newton 62–69 ■ Joseph Black 76–77 ■ Antoine Lavoisier 84 ■ Charles Keeling 294–95

Mathematics compares the These individual waves each move A Fourier series can approximate
most diverse phenomena and uniformly from a peak to a trough. a wave of any shape—even a square
discovers the secret analogies Adding more and more of these one (shown here in pink). Adding more
simple waves together produces sine waves to the series gives a closer
that unite them. increasing complexity that can and closer approximation of the square
Joseph Fourier approximate any other type of wave. The first four approximations in
wave. These infinite series are the series (shown here in black) each
Fourier focused on the idea of now called Fourier series. incorporate an extra sine wave.
waves, and finding a way to
represent them mathematically. Fourier published his idea in
He saw that every wavelike 1807, but it attracted criticism, and
movement, which is what a it was not until 1822 that his work
temperature gradient is, can be was finally accepted. Continuing
approximated mathematically by his study of heat, in 1824, Fourier
adding together simpler waves, examined the difference between
whatever the shape of the wave to the heat that Earth gains from the
be represented. The simpler waves Sun and the heat it loses to space.
that are to be added together are He realized that the reason Earth is
sines and cosines, derived from pleasantly warm, considering how
trigonometry, and can be written far it is from the Sun, is because
out mathematically as a series. gases in its atmosphere trap heat
and stop it from being radiated
back into space—the phenomenon
now called the greenhouse effect.

Today, Fourier analysis is
applied not only to heat transfer
but also to a host of problems
at the cutting edge of science,
ranging from acoustics, electrical
engineering, and optics to
quantum mechanics. ■

Joseph Fourier The son of a tailor, Joseph Fourier to France in 1801, Fourier was
was born in Auxerre, France. made governor of Isère in the
Orphaned at 10, he was taken Alps. In between administrative
into a local convent before going duties overseeing road building
on to a military school, where he and drainage planning, he
excelled at mathematics. France published a groundbreaking
was in the throes of revolution, study of ancient Egypt and
and during the Terror of 1794, he started his studies of heat. He
was briefly imprisoned after died in 1831 after tripping and
falling out with fellow falling down a flight of stairs.
revolutionaries.
Key works
After the Revolution, Fourier
accompanied Napoleon on an 1807 On the Propagation of
expedition to Egypt in 1798. Heat in Solid Bodies
He was made governor of Egypt 1822 The Analytic Theory
and put in charge of the study of of Heat
ancient Egyptian relics. Returning

124

THE ARTIFICIAL PRODUCTION
OF ORGANIC SUBSTANCES
FROM INORGANIC
SUBSTANCES

FRIEDRICH WÖHLER (1800–1882)

IN CONTEXT I n 1807, the Swedish chemist endowed with a “life force” beyond
Jöns Jakob Berzelius suggested the understanding of chemists.
BRANCH that a fundamental difference So it came as a surprise when
Chemistry existed between the chemicals the pioneering experiments of a
involved in living things and all German chemist named Friedrich
BEFORE other chemicals. These unique, Wöhler showed that organic
1770s Antoine Lavoisier and “organic” chemicals, Berzelius chemicals are not unique at all,
others show that water and argued, could only be assembled by but behave according to the same
salt can return to their former living things themselves and, once basic rules as all chemicals.
state after heating, but sugar broken down, could not be remade
or wood cannot. artificially. His idea conformed with We now know that organic
the prevailing theory known as chemicals comprise a multitude of
1807 Jöns Jakob Berzelius “vitalism,” which held that life was molecules based on the element
suggests a fundamental special and that living things were carbon. These carbon-based
difference between organic molecules are indeed essential
and inorganic chemicals. components of life, but many can
be synthesized from inorganic
AFTER chemicals—as Wöhler discovered.
1852 British chemist Edward
Franklin suggests the idea of Widely used in fertilizers, urea is Chemistry rivals
valency, the ability of atoms to rich in nitrogen, which is essential to Wöhler’s breakthrough came about
combine with other atoms. the growth of plants. Synthetic urea, because of a scientific rivalry. In
first made by Wöhler, is now a key raw the early 1820s, Wöhler and fellow
1858 British chemist material in the chemical industry. chemist Justus von Liebig both
Archibald Couper suggests the came up with identical chemical
idea of bonds between atoms, analyses for what seemed to be two
explaining how valency works. very different substances—silver
fulminate, which is explosive, and
1858 Couper and August silver cyanate, which is not. Both
Kekulé propose that organic men assumed that the other had
chemicals are made by chains the wrong results, but after
of bonded carbon atoms with corresponding, they found they
side branches of other atoms. were both right. This group of
compounds led chemists to realize
that substances are defined not just

A CENTURY OF PROGRESS 125

See also: Antoine Lavoisier 84 ■ John Dalton 112–13 ■
Jöns Jakob Berzelius 119 ■ Leo Baekeland 140–41 ■ August Kekulé 160–65

Some chemists think that Yet by mixing two
organic chemicals found ordinary chemicals in the
in living things are unique
lab, we can produce
and can only be made urea—the organic
by living things. chemical in urine.

Organic substances We can make Friedrich Wöhler
are not unique. organic substances
Born in Eschersheim, near
from inorganic Frankfurt in Germany,
substances. Friedrich Wöhler trained in
obstetrics at the University
by the number and kinds of atoms a key component of urine, and has of Heidelberg. But chemistry
in the molecule but also by the the same chemical formula as was his passion and, in 1823,
atoms’ arrangement. The same ammonium cyanate. According to he went to study with Jöns
formula may apply to different Berzelius’s theory, it could be made Jakob Berzelius in Stockholm.
structures with different only by living things—yet Wöhler On his return to Germany, he
properties—these different had synthesized it from inorganic embarked on a remarkable
structures were later named chemicals. Wöhler wrote to Berzelius: and varied career in chemical
isomers by Berzelius. “I must tell you that I can make research and innovation.
urea without the use of kidneys,”
Wöhler and Liebig went on to explaining that urea was in fact an Besides the first artificial
forge a brilliant partnership, but it isomer of ammonium cyanate. synthesis of an organic
was Wöhler alone who, in 1828, substance, Wöhler’s many
stumbled upon the truth about The significance of Wöhler’s discoveries—often made with
organic chemicals. discovery took many years to sink Justus von Liebig—included
in. Even so, it paved the way for the aluminum, beryllium, yttrium,
The Wöhler synthesis development of modern organic titanium, and silicon. He also
Wöhler was mixing silver cyanate chemistry, which not only reveals helped to develop the idea of
with ammonium chloride, expecting how all living things depend on “radicals”—basic molecular
to get ammonium cyanate. Instead, chemical processes, but enables groups from which other
he got a white substance that had the artificial synthesis of valuable substances are built. Although
different properties from ammonium organic chemicals on a commercial later disproved, this theory
cyanate. The same powder appeared scale. In 1907, a synthetic polymer paved the way for today’s
when he mixed lead cyanate with called Bakelite was produced from understanding of how
ammonium hydroxide. Analysis two such chemicals and ushered in molecules assemble. In later
showed the white powder to be the “Age of Plastics” that shaped years, Wöhler became an
urea—an organic substance that is the modern world. ■ authority on the chemistry of
meteorites and helped set up
a factory for purifying nickel.

Key works

1830 Summary of
Inorganic Chemistry
1840 Summary of
Organic Chemistry

126

WINDS NEVER
BLOW IN A
STRAIGHT LINE

GASPARD-GUSTAVE DE CORIOLIS (1792–1843)

IN CONTEXT A ir and ocean currents do Earth’s rotation causes winds to be
not flow in straight lines. deflected to the right in the northern
BRANCH As the currents move, hemisphere and left in the southern.
Meteorology they are deflected to the right in
the northern hemisphere, and Initial direction Deflected
BEFORE to the left in the southern. In the right
1684 Isaac Newton introduces 1830s, French scientist Gaspard-
the idea of centripetal force, Gustave de Coriolis discovered the
stating that any motion in a principle behind this effect, now
curved path must be the result known as the Coriolis effect.
of a force acting on it.
Deflected by rotation Deflected left Initial direction
1735 George Hadley suggests Coriolis got his ideas from
that trade winds blow toward studying turning waterwheels, simply blow straight from high
the equator because Earth’s but meteorologists later realized pressure areas to low pressure
rotation deflects air currents. that the ideas apply to the way areas. The wind direction is in fact
winds and ocean currents move. a balance between the pull of low
AFTER pressure and the Coriolis deflection.
1851 Léon Foucault shows Coriolis showed how, when an This is mostly why winds circle
how the swing of a pendulum object is moving across a rotating counterclockwise into low pressure
is deflected by Earth’s rotation. surface, its momentum seems to zones in the northern hemisphere,
carry it on a curved path. Imagine and clockwise in the southern
1856 US meteorologist William throwing a ball out from the center hemisphere. Similarly, ocean
Ferrel shows that winds blow of a spinning merry-go-round. The surface currents circulate in
parallel to isobars—lines that ball appears to curve around—even giant loops or gyres, clockwise
connect points of equal though to anyone watching from in the northern hemisphere and
atmospheric pressure. outside the merry-go-round it is counterclockwise in the south. ■
actually moving in a straight line.
1857 Dutch meteorologist
Christophorus Buys Ballot Winds on the rotating Earth are
formulates a rule stating that deflected in the same way. Without
if the wind is blowing on your the Coriolis effect, winds would
back, an area of low pressure
is to your left. See also: George Hadley 80 ■ Robert FitzRoy 150–55

A CENTURY OF PROGRESS 127

ON THE COLORED
LIGHT OF THE
BINARY STARS

CHRISTIAN DOPPLER (1803–1853)

IN CONTEXT T he color of light depends The colors of stars are now
on its frequency, which known to be mainly due to their
BRANCH is the number of waves temperature (the hotter the star,
Physics per second. If something moving the more blue it appears), but the
toward us is emitting waves, the movement of some stars can be
BEFORE second wave will have a shorter detected through Doppler shifts.
1677 Ole Rømer estimates distance to travel than the first Binary stars are pairs of stars
the speed of light by studying wave, so it will arrive sooner than it orbiting each other. Their rotation
Jupiter’s moons. would if the source were stationary. causes an alternating redshift and
Thus the frequency of waves blueshift in the light they emit. ■
AFTER increases if the source and receiver
1840s Dutch meteorologist are getting closer to each other, and The heavens presented an
Christophorus Buys Ballot decreases if they are moving apart. extraordinary appearance, for
applies the Doppler shift to This effect applies to all types of all the stars directly behind
sound waves, as does French wave, including sound, and is me were now deep red, while
physicist Hippolyte Fizeau to responsible for the changing pitch
electromagnetic waves. of a siren as an ambulance passes. those directly ahead were
violet. Rubies lay behind me,
1868 British astronomer To the naked eye, most stars
William Huggins uses redshift appear to be white, but through amethysts ahead of me.
to find the velocity of a star. a telescope many can be seen to Olaf Stapledon
be red, yellow, or blue. In 1842, an
1929 Edwin Hubble relates Austrian physicist named Christian From his novel, Star Maker (1937)
the redshift of galaxies to their Doppler suggested that the red color
distance from Earth, showing of some stars is due to the fact that
the expansion of the universe. they are moving away from the
Earth, which would shift their light
1988 The first extrasolar to longer wavelengths. Since the
planet is detected, using the longest wavelength of visible light is
Doppler shift of light from red, this became known as redshift
the star that it orbits—the (as illustrated on p.241).
star appears to “wobble” as
the planet’s gravitational See also: Ole Rømer 58–59 ■ Edwin Hubble 236–41 ■ Geoffrey Marcy 327
pull disrupts its rotation.

128

THE GLACIER
WAS GOD’S
GREAT PLOUGH

LOUIS AGASSIZ (1807–1873)

IN CONTEXT Retreating glaciers leave particular features behind them
in the landscape.
BRANCH
Earth science These features are found in areas where there are no glaciers.

BEFORE There must have been glaciers in these
1824 Norwegian Jens Esmark places some time in the past.
suggests that glaciers are
responsible for the creation of W hen glaciers sweep which is the usual way that rocks
fjords, erratics, and moraines. across a landscape, are carried across a landscape. A
they leave signature rock of a different kind from rocks
1830 Charles Lyell argues that features behind them. Glaciers around it, therefore, is a telltale
the laws of nature have always can scour rocks flat or leave them sign that a glacier once passed
been the same, so the clues to smoothly rounded, often with by. Another is the presence of
the past lie in the present. striations (scratch marks) showing moraines in valleys. These are piles
the direction in which the ice of boulders that were pushed aside
1835 Swiss geologist Jean moved. They also leave behind when the glacier was growing, and
de Charpentier argues that erratics—boulders that have been left behind when it retreated.
erratics near Lake Geneva carried long distances by the ice.
were transported by ice from These can usually be identified Riddle of the rocks
the Mont Blanc area in an because their composition is Geologists in the 19th century
“Alpine glaciation.” different from the rocks on which recognized such features as
they lie. Many erratics are too large striations, erratics, and moraines
AFTER to have been moved by rivers, as evidence of glaciers. What they
1875 Scottish scientist James
Croll argues that variations in
Earth’s orbit could explain the
temperature changes that
cause an ice age.

1938 Serbian physicist Milutin
Milankovic relates changes in
climate to periodic changes
in Earth’s orbit.

A CENTURY OF PROGRESS 129

See also: WIlliam Smith 115 ■ Alfred Wegener 222–23

could not explain was why such Agassiz wanted to convince others. Louis Agassiz
features were found in areas on He had met William Buckland, a
Earth that had no glaciers. One prominent English geologist, while Born in a small Swiss village
theory argued that rocks were excavating fossil fishes in the Old in 1807, Louis Agassiz studied
moved by repeated flooding. Floods Red Sandstone rocks in the Alps. to be a physician, but became
could explain the “boulder drift” When Agassiz showed him the a professor of natural history
(the sands, clays, and gravels that evidence for his theory of an ice at the University of Neuchâtel.
included erratic boulders) that age, Buckland was convinced, His first scientific work, under
overlay much of the bedrock of and in 1840 the two men toured the French naturalist Georges
Europe. The material might have Scotland to look for evidence of Cuvier, involved classifying
been deposited when the last flood glaciation there. After the tour, freshwater fish from Brazil,
retreated. The largest erratics could Agassiz presented his ideas to and Agassiz went on to
have been caught up in icebergs, the Geological Society of London. undertake extensive work
which deposited the rocks when Although he had convinced on fossilized fish. In the late
they melted. But the theory could Buckland and Charles Lyell—two 1830s, his interests spread
not explain all of the features. of the leading geologists of the to glaciers and zoological
day—the other members of the classification. In 1847, he took
The ice age revealed society were unimpressed. A nearly a post at Harvard University
During the 1830s, Swiss geologist global glaciation seemed no more in the US.
Louis Agassiz spent several probable than a global flood.
vacations in the European Alps However, the idea of ice ages Agassiz never accepted
studying glaciers and their valleys. gradually gained acceptance, and Darwin’s theory of evolution,
He realized that glacial features today there is evidence from many believing that species were
everywhere, not just in the Alps, different fields of geology that ice “ideas in the mind of God”
could be explained if Earth had has covered much of Earth’s and that all species had been
once been covered in far more surface many times in the past. ■ created for the regions they
ice than at present. The glaciers inhabited. He advocated
of today must be the remnants of Agassiz was the first to suggest “polygenism,” a belief that
ice sheets that had at one time that large erratics, such as these in the different human races did not
covered most of the globe. But Caher Valley of Ireland, were deposited share a common ancestor, but
before he published his theory by ancient glaciers. were created separately by
God. In recent years, his
reputation has been tarnished
by his apparent advocacy of
racist ideas.

Key works

1840 Study on Glaciers
1842–46 Nomenclator
Zoologicus

NATURE

CAN BE REPRESENTED

AS ONE GREAT

WHOLE

ALEXANDER VON HUMBOLDT (1769–1859)



132 ALEXANDER VON HUMBOLDT

IN CONTEXT T he study of the The principal impulse by
interrelationship between which I was directed was the
BRANCH the animate and inanimate
BIOLOGY world, known as ecology, only earnest endeavour to
became a subject of rigorous and comprehend the phenomena
BEFORE methodical scientific investigation of physical objects in their
5th–4th century BCE Ancient over the last 150 years. The term general connection, and to
Greek writers observe the “ecology” was coined in 1866 by represent nature as one great
web of interrelationships the German evolutionary biologist, whole, moved and animated
between plants, animals, Ernst Haeckel, and is derived from
and their environment. the Greek words oikos, meaning by internal forces.
house or dwelling place, and logos, Alexander von Humboldt
AFTER meaning study or discourse. But
1866 Ernst Haeckel coins it is an earlier German polymath by ancient Greek writers, such as
the word “ecology.” named Alexander von Humboldt Herodotus in the 5th century BCE.
who is regarded as the pioneer of In one of the first accounts of
1895 Eugenius Warming modern ecological thinking. interdependence, technically
publishes the first university known as mutualism, he describes
course book on ecology. Through extensive expeditions crocodiles on the Nile River in
and writings, Humboldt promoted Egypt opening their mouths to
1935 Alfred Tansley coins the a new approach to science. He allow birds to pick their teeth clean.
word “ecosystem.” sought to understand nature as
a unified whole, by interrelating A century later, observations
1962 Rachel Carson warns of all of the physical sciences and by the Greek philosopher Aristotle
the dangers of pesticides in employing the latest scientific and his pupil Theophrastus on
Silent Spring. equipment, exhaustive observation, species’ migration, distribution,
and meticulous analysis of data on and behavior provided an early
1969 Friends of the Earth and an unprecedented scale. version of the concept of the
Greenpeace are established. ecological niche—the particular
The crocodile’s teeth place in nature that shapes and is
1972 James Lovelock’s Gaia Although Humboldt’s holistic shaped by a species’ way of life.
hypothesis presents Earth as approach was new, the concept Theophrastus studied and wrote
a single organism. of ecology developed from early extensively on plants, realizing the
investigations of natural history importance of climate and soils to
their growth and distribution. Their
ideas influenced natural philosophy
for the next 2,000 years.

Humboldt’s team climbed Mexico’s
Jorullo volcano in 1803, just 44 years
after it first appeared. Humboldt linked
geology to meteorology and biology by
studying where different plants lived.

A CENTURY OF PROGRESS 133

See also: Jean-Baptiste Lamarck 118 ■ Charles Darwin 142–49 ■ James Lovelock 315

Nature’s unifying forces as a means of characterizing and showed the global application
Humboldt’s approach to nature mapping the global environment, of these zones by comparing the
followed in the late 18th-century especially the climate, and then Andean zones with those of the
Romantic tradition that reacted comparing the climatic conditions European Alps, Pyrenees, Lapland,
to rationalism by insisting on the in various countries. Tenerife, and the Asian Himalayas.
value of senses, observation, and
experience in understanding Humboldt was also one of Defining ecology
the world as a whole. Like his the first scientists to study how When Haeckel coined the word
contemporaries, the poets Johann physical conditions—such as “ecology,” he too was following in
Wolfgang von Goethe and Friedrich climate, altitude, latitude, and the tradition of viewing a Gestalt
Schiller, Humboldt promoted the soils—affected the distribution of (unity) of the living and inanimate
idea of the unity (or Gestalt in life. With Bonpland’s assistance, he world. An enthusiastic evolutionist,
German) of nature—and of natural mapped the changes in flora and he was inspired by Charles Darwin,
philosophy and the humanities. His fauna between sea level and high whose publication of On the Origin
studies ranged from anatomy and altitude in the Andes. In 1805, of Species in 1859 banished the
astronomy to mineralogy and the year after his return from the notion of Earth as an immutable
botany, commerce, and linguistics, Americas, he published a now- world. Haeckel questioned the role
and provided him with the breadth celebrated work on the geography of natural selection, but believed
of knowledge necessary for his of the area, summarizing the that the environment played an
exploration of the natural world interconnectedness of nature and important role in both evolution
beyond the confines of Europe. illustrating the altitudinal zones of and ecology. ❯❯
vegetation. Years later, in 1851, he
As Humboldt explained, “The
sight of exotic plants, even of dried Ecology is the study of all the interactions
specimens in a herbarium, fired my between organisms and their environment that
imagination and I longed to see
the tropical vegetation in southern determine their distribution and abundance.
countries with my own eyes.”
His five-year exploration of Latin These interactions must include…
America with the French botanist
Aimé Bonpland was his most …biotic factors, such …abiotic factors, such
important expedition. Setting out as human activity and animal as climate, soils, and the
in June 1799, he declared, “I shall
collect plants and fossils, and make and plant communities. hydrological cycle.
astronomical observations with the
best of instruments. Yet this is not Nature can be represented
the main purpose of my journey. I as one great whole.
shall endeavor to discover how
nature’s forces act upon one another
and in what manner the geographic
environment exerts its influence on
animals and plants. In short, I must
find out about the harmony in
nature.” And he did just that.

Among many other projects,
Humboldt measured ocean water
temperature and suggested the use
of “isolines,” or isothermal lines, to
connect points of equal temperature

134 ALEXANDER VON HUMBOLDT

By the end of the 19th century, the encompassing all those factors that This whole chain of
first university course in ecology influence it—both biotic (living poisoning, then, seems
was being taught by the Danish organisms) and abiotic (nonliving to rest on a base of minute
botanist Eugenius Warming, factors such as soil, water, air, plants which must have been
who also wrote the first ecology temperature, and sunlight). The the original concentrators.
textbook Plantesamfund (Plant scope of modern ecology ranges
Ecology) in 1895. From Humboldt’s from the individual organism to Rachel Carson
pioneering work, Warming populations of individuals of the
developed the global geographical same species, and the community, which successive communities
subdivision of plant distribution made up of populations that share of different species adjust to one
known as biomes, such as the a particular environment. another to form a tightly integrated
tropical rain-forest biome, which and interdependent unit, similar
are largely based on the interaction Many of the basic terms and to the organs of a body. Clements’
of plants with the environment, concepts of ecology came from the metaphor of the community as a
especially climate. work of several pioneer ecologists “complex organism” was criticized
in the first few decades of the 20th at first but influenced later thinking.
Individuals and community century. The formal concept of the
Early in the 20th century, the biological community was first The idea of further ecological
modern definition of ecology developed in 1916 by the American integration at a higher level than
developed as the scientific study botanist Frederic Clements. He the community was introduced
of the interactions that determine believed that the plants of a given in 1935 with the concept of the
the distribution and abundance area develop a succession of ecosystem, developed by the
of organisms. These interactions communities over time, from English botanist Arthur Tansley.
include an organism’s environment, an initial pioneer community to an An ecosystem consists of both
optimal climax community within living and nonliving elements.
Their interaction forms a stable
A food chain transfers energy from primary unit with a sustaining flow of
producers (plants and algae that convert the Sun’s energy from the environmental
energy into food energy) to consumer organisms that to the living part (through the food
eat the plants (such as rabbits and other herbivores), chain) and can operate on all
and then to the predators that feed on the consumers. scales, from a puddle to an ocean
or the whole planet.
Lion, an apex Jackal
predator (not preyed Studies of animal communities
Goat by the English zoologist Charles
on by others) Elton led him to develop in 1927 the
Green concept of the food chain and food
Kite plants cycle, subsequently known as the
Wild cat “food web.” A food chain is formed
Rabbit by the transfer of energy through an
Owl Mouse ecosystem from primary producers
(such as green plants on land)
Snake through a series of consuming

A CENTURY OF PROGRESS 135

Rachel Carson (far right) made a
significant contribution to the science
and public understanding of ecology
by drawing attention to the destructive
impact of pollution on the environment.

organisms. Elton also recognized harmful effects on the environment renewable energy, organic foods,
that particular groups of organisms of man-made chemicals such as recycling, and sustainability, were
occupied certain niches in the food the pesticide DDT. The first image all on the political agenda in both
chain for periods of time. Elton’s of Earth seen from space, taken North America and Europe, and
niches include not only the habitats by Apollo 8 astronauts in 1968, national conservation agencies
but also the resources upon which awakened public awareness of were established based on the
the occupying organisms rely for the planet’s fragility. In 1969, the science of ecology. Recent decades
sustenance. The dynamics of organizations Friends of the Earth have seen growing concern over
energy transfer through trophic and Greenpeace were established, global climate change and its
(feeding) levels were studied by with the mission to “ensure the impact on the environment and
the American ecologists Raymond ability of the Earth to nurture life present ecosystems, many of
Lindeman and Robert MacArthur, in all its diversity.” Environmental which are already threatened
whose mathematical models protection, along with clean and from human activity. ■
helped change ecology from
primarily a descriptive science
into an experimental one.

The green movement
A boom in popular and scientific
interest in ecology in the 1960s
and 1970s led to the development
of the environmental movement
with a whole range of concerns,
stimulated by powerful advocates
such as the American marine
biologist Rachel Carson. Her 1962
book Silent Spring documented the

Alexander von Born in Berlin to a wealthy and On his return, Humboldt was
Humboldt well-connected family, Humboldt honored across Europe. Based in
studied finance at the University Paris, he took 21 years to process
of Frankfurt, natural history and and publish his data in over 30
linguistics in Göttingen, language volumes, and then synthesized
and commerce in Hamburg, his ideas in four volumes titled
geology in Freiburg, and anatomy Kosmos. A fifth volume was
in Jena. The death of his mother in completed after his death in
1796 provided Humboldt with the Berlin at 89. Darwin called him
means to fund an expedition to “the greatest scientific traveller
the Americas from 1799 to 1804, who ever lived.”
accompanied by botanist Aimé
Bonpland. Using the latest Key works
scientific equipment, Humboldt
measured everything from plants 1825 Journey to the Equinoctial
to population statistics and Regions of the New Continent
minerals to meteorology. 1845–1862 Kosmos

136

LIGHT TRAVELS MORE
SLOWLY IN WATER
THAN IN AIR

LÉON FOUCAULT (1819–1868)

IN CONTEXT Is light a stream Whichever it is,
of particles or a wave? light takes time
BRANCH
Physics to travel.

BEFORE Foucault found Newton thought
1676 Ole Rømer makes the that light travels light particles would
first successful estimate of the speed up going from air
speed of light, using eclipses more slowly in to water, while Huygens
of Io, one of Jupiter’s moons. water than in air. thought waves would

1690 Christiaan Huygens slow down.
publishes his Treatise on
Light, in which he proposes Therefore, light must
that light is a type of wave. travel in waves.

1704 Isaac Newton’s Opticks I n the 17th century, scientists published his theory of light
suggests that light is a stream began to investigate light, as a stream of “corpuscles,” or
of “corpuscles.” and whether it had a finite, particles. Newton’s explanation
measurable speed. In 1690, for refraction—the bending of a
AFTER Christiaan Huygens published his beam of light as it passes from one
1864 James Clerk Maxwell theory that light is a pressure wave, transparent material to another—
realizes that the speed of moving in a mysterious fluid called assumed that light travels faster
electromagnetic waves is so ether. Huygens thought of light as after it passes from air into water.
nearly the same as the speed a longitudinal wave, and predicted
of light that light must be a that the wave would travel more Estimates for the speed of light
form of electromagnetic wave. slowly through glass or water than relied on astronomical phenomena,
through air. In 1704, Isaac Newton showing how fast light travels
1879–83 German-born US through space. The first terrestrial
physicist Albert Michelson
refines Foucault’s method and
obtains a measurement for the
speed of light (through air) that
is very close to today’s value.

A CENTURY OF PROGRESS 137

See also: Christiaan Huygens 50–51 ■ Ole Rømer 58–59 ■ Isaac Newton 62–69 ■ Thomas Young 110–11 ■
James Clerk Maxwell 180–85 ■ Albert Einstein 214–21 ■ Richard Feynman 272–73

Above all we must be Contradicting Newton argued, light could not be a
accurate, and it is an In 1850, Fizeau collaborated with particle, and the experiment was
obligation which we intend fellow physicist León Foucault, who viewed at the time as a refutation
to fulfill scrupulously. adapted his apparatus—and made of Newton’s theory of corpuscles.
it much smaller—by reflecting the Foucault refined his apparatus
Léon Foucault beam of light off a rotating mirror further, and in 1862, measured the
instead of passing it through the speed of light in air as 185,168
measurement was carried out by cogwheel. Light shining at the miles/s (298,000 km/s)—remarkably
French physicist Hippolyte Fizeau rotating mirror would only be close to today’s value of 186,282
in 1849. A beam of light was shone reflected toward the distant mirror miles/s (299,792 km/s). ■
through a gap between the teeth of when the rotating mirror was at
a rotating cogwheel. That light was the correct angle. Light returning Tube of water (for the
then reflected by a mirror that was from the fixed mirror was reflected speed of light in water)
positioned 5 miles (8 km) away, and by the rotating mirror again, but
passed back through the next gap since this mirror had moved while Rotating
between the wheel’s teeth. Taking the light was traveling, it was not mirror
the precise speed of rotation that reflected directly back toward the
allowed this to happen, together source. The speed of light could Fixed
with time and distance, Fizeau now be calculated from the angle mirror
calculated the speed of light as between the light going to and
194,489 miles/s (313,000 km/s). from the rotating mirror and the Light
speed of rotation of the mirror. source

The speed of light in water Reflected light
could be measured by putting
a tube of water in the apparatus In Foucault’s experiment, the
between the rotating and speed of light was calculated from
stationary mirrors. Using this the difference in angle as a beam of
apparatus, Foucault established light reflected back and forth between
that light traveled more slowly in a rotating mirror and a fixed mirror.
water than in air. As such, he

Léon Foucault Born in Paris, France, Léon a pendulum in 1851 and later a
Foucault was educated mainly gyroscope. Although he had no
at home before entering medical formal training in science, a post
school, where he studied under was created for Foucault at the
the bacteriologist Alfred Donné. Imperial Observatory in Paris.
Since he could not bear the sight He was also made a member of
of blood, Foucault soon gave up several scientific societies, and
his studies, became Donné’s is one of 72 French scientists
laboratory assistant, and devised named on the Eiffel Tower.
a way of taking photographs
through a microscope—he later Key works
teamed up with Hippolyte Fizeau
to take the first ever photograph of 1851 Demonstration of Physical
the Sun. In addition to measuring Movement of Rotation of the
the speed of light, Foucault is best Earth by Means of the Pendulum
known for providing experimental 1853 On the Relative Velocities
evidence of Earth’s rotation, using of the Light in Air and in Water

138

LIVING FORCE MAY BE
CONVERTED INTO HEAT

JAMES JOULE (1818–1889)

IN CONTEXT T he principle of the was ever lost in this conversion.
conservation of energy His ideas were largely ignored
BRANCH states that energy is never until 1847, when German physicist
Physics lost but only changed in form. But Hermann Helmholtz published a
in the 1840s, scientists had only a paper summarizing the theory of
BEFORE vague idea of what energy was. the conservation of energy, and
1749 French mathematician It was a British brewer’s son, Joule then presented his work at
Émilie du Châtelet derives her James Joule, who showed that the British Association in Oxford.
law of the conservation of heat, mechanical movement, and The standard unit of energy, a joule,
energy from Newton’s laws. electricity are interchangeable is named after him. ■
forms of energy, and that when
1824 French engineer Sadi one is changed to another the
Carnot states that there are no total energy remains the same.
reversible processes in nature,
paving the way for the second Converting energy In Joule’s experiment, a falling
law of thermodynamics. Joule began his experiments in weight drove a paddle that turned
a laboratory in the family home. inside a bucket of water. The energy of
1834 French physicist Émile In 1841, he figured out how much the movement was changed into heat.
Clapeyron develops Carnot’s heat an electric current generates.
work, stating a version of the He experimented with converting
second law of thermodynamics. mechanical movement into heat,
and developed an experiment in
AFTER which a falling weight turns a
1850 German physicist Rudolf paddle wheel in water, heating
Clausius gives the first clear the water. By measuring the rise
statement of the first and second in temperature of the water, Joule
laws of thermodynamics. was able to figure out the exact
amount of heat a certain amount of
1854 Scottish engineer mechanical work would create. He
William Rankine adds the went on to assert that no energy
concept that is later named
entropy (a measure of disorder) See also: Isaac Newton 62–69 ■ Joseph Black 76–77 ■ Joseph Fourier 122–23
in the transformation of energy.

A CENTURY OF PROGRESS 139

STATISTICAL
ANALYSIS OF
MOLECULAR
MOVEMENT

LUDWIG BOLTZMANN (1844–1906)

IN CONTEXT B y the middle of the In the early 18th century, Swiss
19th century, atoms and physicist Daniel Bernoulli had
BRANCH molecules had become suggested that gases are made of a
Physics central ideas in chemistry, and multitude of moving molecules. It is
most scientists understood that their impact that creates pressure
BEFORE they were the key to the identity and their kinetic energy (the energy
1738 Daniel Bernoulli and behavior of elements and of their movement) that creates
suggests that gases are compounds. Few thought they heat. In the 1840s and 1850s,
made of moving molecules. had much relevance to physics, scientists had begun to realize that
but in the 1880s, Austrian physicist the properties of gases reflect the
1827 Scottish botanist Ludwig Boltzmann developed the average movement of the countless
Robert Brown identifies the kinetic theory of gases, putting particles. In 1859, James Clerk
movement of pollen in water, atoms and molecules right at the Maxwell calculated the speed
which becomes known as heart of physics, too. of molecules and how far they
Brownian motion. traveled before colliding, showing
that temperature is a measure of
1845 Scottish physicist John the average speed of the molecules.
Waterston describes how
energy among gas molecules Available energy is the Centrality of statistics
is distributed according to main object at stake in the Boltzmann revealed how important
statistical rules. struggle for existence and the statistics are. He showed that the
the evolution of the world. properties of matter are simply a
1857 James Clerk Maxwell combination of the basic laws of
calculates the mean speed Ludwig Boltzmann motion and the statistical rules of
of molecules and the mean probability. Following this principle,
distance between collisions. he calculated a number now called
the Boltzmann constant, providing
AFTER a formula linking the pressure and
1905 Albert Einstein volume of a gas to the number and
analyzes Brownian motion energy of its molecules. ■
mathematically, showing
how it is the result of the See also: John Dalton 112–13 ■ James Joule 138 ■
impact of molecules. James Clerk Maxwell 180–85 ■ Albert Einstein 214–21

140

PLASTIC IS NOT
WHAT I MEANT
TO INVENT

LEO BAEKELAND (1863–1944)

IN CONTEXT T he discovery of synthetic Baekeland created one of the first
plastics in the 19th century commercially successful plastics,
BRANCH opened the way to the now known as Bakelite.
Chemistry creation of a huge range of solid
materials unlike anything that What gives plastic its special
BEFORE had ever been known before— quality is the shape of its
1839 Berlin apothecary light, noncorroding, and capable molecules. With only a few
Eduard Simon distils styrol of being molded into almost any exceptions, plastics are made
resin from the Turkish sweet- imaginable shape. While plastics from long organic molecules, known
gum tree. A century later, this can occur naturally, all of the as polymers, strung together
is developed into polystyrene plastics now in widespread use are from many smaller molecules, or
by the German IG Farben entirely synthetic. In 1907, Belgian- monomers. A few polymers occur
company. born American inventor Leo naturally, such as cellulose, the
main woody substance in plants.
1862 Alexander Parkes
develops the first synthetic Materials made from long Shellac, a resin used in
plastic, Parkesine. molecules called polymers varnish, is a naturally
occurring polymer.
1869 American John Hyatt have special qualities.
creates celluloid, which is
soon used instead of ivory This artificial polymer It is possible to make
to make billiard balls. can be used to produce artificial shellac by
strong, hard moldable
AFTER materials, called plastics. treating coal tar.
1933 British chemists Eric
Fawcett and Reginald Gibson Plastic is not what I meant to invent.
of the ICI company create the
first practical polythene.

1954 Italian Giulio Natta
and German Karl Rehn
independently invent
polypropylene, now the
most widely used plastic.

A CENTURY OF PROGRESS 141

See also: Friedrich Wöhler 124–25 ■ August Kekulé 160–65 ■
Linus Pauling 254–59 ■ Harry Kroto 320–21

I was trying to make could make a kind of shellac. In Leo Baekeland
something really hard, but 1907, he added various kinds of
then I thought I should make powder to this resin and found Leo Baekeland was born in
something really soft instead, that he could create a remarkable Ghent in Belgium and studied
that could be moulded into hard, moldable plastic. at the university there. In
different shapes. That was how 1889, he became associate
I came up with the first plastic. Chemically this plastic is known professor of chemistry and
as polyoxybenzylmethylenglycolan- married Celine Swarts.
Leo Baekeland hydride, but Baekeland called it While the young couple
simply Bakelite. Bakelite was a were on honeymoon in
Although the molecules of natural “thermoset” plastic—plastic that New York, Baekeland met
polymers were far too complex holds its shape after being heated. Richard Anthony, head of a
to figure out in the 1800s, some Due to its properties of electrical well-known photographic
scientists began to explore ways insulation and heat resistance, company. Anthony was so
of making them synthetically Bakelite was soon being used to impressed by Baekeland’s
from chemical reactions. In 1862, make radios, telephones, and work with photographic
British chemist Alexander Parkes electrical insulators. Many more processes that he hired him
created a synthetic form of cellulose, uses were quickly found for it. as a consulting chemist.
which he called Parkesine. A few Baekeland moved to the
years later, American John Hyatt Today, there are thousands US and was soon in
developed another, which became of synthetic plastics, including business for himself.
known as celluloid. Plexiglass, polythene, low-density
polyethylene, and cellophane, Baekeland invented the
each with its own properties and first photographic papers,
uses. The majority are based known as Velox, before
on hydrocarbons (chemicals developing Bakelite, which
made from hydrogen and carbon) made him rich. He is credited
derived from oil or natural gas. with many inventions besides
However, in recent decades, plastic, registering more than
carbon fibers, nanotubes and 50 patents in total. In later
other materials have been added life, he became an eccentric
to create superlight, superstrong recluse, eating food only from
plastic materials such as Kevlar. ■ tin cans. He died in 1944 and
is buried in Sleepy Hollow
Imitating nature Heat-resistant and nonconductive Cemetery, New York.
After developing the world’s first of electricity, Bakelite was an ideal
photographic paper in the 1890s, material to use for the casings of Key work
Baekeland sold the idea to Kodak electrical goods such as telephones
and used the money to buy a house and radios. 1909 Paper on Bakelite
equipped with its own laboratory. read to the American
Here, he experimented with ways Chemical Society
of creating synthetic shellac.
Shellac is a resin secreted by the
female lac beetle. It is a natural
polymer that was used to give
furniture and other objects a tough,
shiny coat. Baekeland found that
by treating phenol resin made from
coal tar with formaldehyde, he

I HAVE CALLED

THIS PRINCIPLE

NATURAL

SELECTION

CHARLES DARWIN (1809–1882)



144 CHARLES DARWIN Most organisms produce more offspring than
can survive due to constraints such as lack of
IN CONTEXT
food and living space.
BRANCH
Biology Offspring vary from each other in many ways.

BEFORE Variation means some offspring are better
1794 Erasmus Darwin suited or adapted to the struggle for survival.
(Charles’s grandfather)
recounts his vision of If these individuals pass on the advantageous
evolution in Zoonomia. traits to their offspring, these also survive.

1809 Jean-Baptiste Lamarck I have called this principle “natural selection.”
proposes a form of evolution
through the inheritance of
acquired characteristics.

AFTER
1937 Theodosius Dobzhansky
publishes his experimental
evidence for the genetic basis
of evolution.

1942 Ernst Mayr defines the
concept of species through
populations that reproduce
only with one another.

1972 Niles Eldredge and
Stephen Jay Gould propose
that evolution occurs mainly in
short bursts interspersed with
periods of relative stability.

T he British naturalist Preservation of Favoured Races in biology, providing a simple, but
Charles Darwin was by no the Struggle for Life, published in immensely powerful, explanation
means the first scientist London in 1859. Darwin described of life forms both past and present.
to suggest that plants, animals, and the book as “one long argument.”
other organisms are not fixed and Darwin was acutely aware of
unchanging—or, to use the popular “Confessing a murder” the potential blasphemy in his
word of the time, “immutable.” Like On the Origin of Species met with work during the decades he spent
others before him, Darwin proposed academic and popular opposition. writing it. Fifteen years before
that species of organisms change, It made no mention of religious publication, he explained to his
or evolve, through time. His great doctrine, which insisted that confidant, the botanist Joseph
contribution was to show how species were indeed fixed and Hooker, that his theory required
evolution took place by a process immutable and designed by God. no God or unchanging species:
he termed natural selection. He But gradually the ideas in the book “At last gleams of light have come,
laid out his central idea in his changed the scientific perspective & I am almost convinced (quite
book On the Origin of Species by on the natural world. Its core notion contrary to opinion I started with)
Means of Natural Selection, or the forms the basis for all modern that species are not (it is like
confessing a murder) immutable.”

A CENTURY OF PROGRESS 145

See also: James Hutton 96–101 ■ Jean-Baptiste Lamarck 118 ■ Gregor Mendel 166–71 ■ Thomas Henry Huxley 172–73 ■
Thomas Hunt Morgan 224–25 ■ Barbara McClintock 271 ■ James Watson and Francis Crick 276–83 ■ Michael Syvanen 318–19

Creation is not an event that In 1796, the French naturalist Philosophie Zoologique of 1809
happened in 4004 BCE; it is a Georges Cuvier recognized that articulated what was perhaps the
certain fossils, such as those first reasoned theory of evolution.
process that began some of mammoths or giant sloths, were He theorized that living beings
10 billion years ago and the remains of animals that had evolved from simple beginnings
become extinct. He reconciled this through increasingly sophisticated
is still under way. with his religious belief by invoking stages, due to a “complexifying
Theodosius Dobzhansky catastrophes such as the Flood force.” They faced environmental
depicted in the Bible. Each disaster challenges on their body physiques,
Darwin’s approach to evolution, swept away a whole assortment of and from this came the idea of
like the rest of his wide-ranging living things; God then replenished use and disuse in an individual:
work in natural history, was Earth with new species. Between “More frequent and continuous use
cautious, careful, and deliberate. each disaster, each species of any organ gradually strengthens,
He proceeded step by step, remained fixed and immutable. develops and enlarges that organ…
amassing great quantities of This theory was known as while the permanent disuse of
evidence along the way. Over “catastrophism” and it became any organ imperceptibly weakens
almost 30 years, he integrated widely known following the and deteriorates it…until it finally
his extensive knowledge of fossils, publication of Cuvier’s Preliminary disappears.” The organ’s greater
geology, plants, animals, and Discourse in 1813. power was then passed to
selective breeding, with concepts offspring, a phenomenon that
from demography, economics, and However, at the time Cuvier became known as inheritance
many other fields. The resulting was writing, various ideas based of acquired characteristics.
theory of evolution by natural on evolution were already in
selection is regarded as one of the circulation. Erasmus Darwin, Although his theory came to be
greatest scientific advances ever. the free-thinking grandfather largely discounted, Lamarck was
of Charles, proposed an early, later praised by Darwin for having
The role of God idiosyncratic theory. More opened up the possibility that
In the early 19th century, fossils influential were the ideas of change did not occur as a result of
were widely discussed in Victorian Jean-Baptiste Lamarck, professor what Darwin disparagingly termed
society. Some regarded them as of zoology at France’s National “miraculous interposition.”
naturally formed rock shapes, Museum of Natural History. His
and nothing to do with living Adventures of the Beagle
organisms. Others saw them as Darwin had plenty of time to muse
the handiwork of the Creator, put on the immutability of species
on Earth to test believers. Or they during an around-the-world voyage
thought that they were the remains aboard the survey ship HMS
of organisms still alive somewhere Beagle, in 1831–36, under captain
in the world, since God had created Robert FitzRoy. As expedition
living things in perfection. scientist, Darwin was charged
with collecting all types of fossil,
plant, and animal specimens, and
sending them back to Britain from
each port of call. ❯❯

By studying the fossil record, Georges
Cuvier established that species had
become extinct. But he believed that
the evidence pointed to a series of
catastrophes, not gradual change.

146 CHARLES DARWIN

This epic voyage opened the eyes modified for different ends.” This Natural selection is the…
of the young Darwin, still only in his was one of the first clear, public principle by which slight
twenties, to the incredible variety formulations of where his thoughts variation (of a trait), if useful,
of life. Wherever the Beagle docked, on evolution were heading.
Darwin keenly observed all aspects is preserved.
of nature. In 1835, he described Comparing species Charles Darwin
and collected a group of small, Darwin’s finches, as the Galápagos
insignificant birds on the specimens became known, were tortoises, Falkland Island foxes,
Galápagos Islands, a Pacific Ocean not the only trigger for his work on and other species supported these
archipelago 560 miles (900 km) evolution. In fact, his thoughts had early conclusions. But Darwin
west of Ecuador. He thought there been mounting throughout the was sensitive about where such
were nine species, six being finches. Beagle’s voyage, and especially blasphemous ideas would lead:
during his visit to the Galápagos. “Such facts would undermine the
After his return to England, He was fascinated by the giant stability of species.”
Darwin organized his mass of tortoises he saw, and by the way
data and oversaw a multivolume, the shapes of their shells differed Other parts of the jigsaw
multiauthor report, The Zoology of subtly from island to island. On his way to South America in
the Voyage of HMS Beagle. In the He was also impressed by the 1831, Darwin had read the first
volume on birds, the renowned species of mockingbirds. They, volume of Charles Lyell’s Principles
ornithologist John Gould declared too, varied between the islands, of Geology. Lyell argued against
that there were in fact 13 species yet they also had similarities not Cuvier’s catastrophism history
in Darwin’s specimens, all of them only among themselves, but with and his theory of fossil formation.
finches. Within the group, however, species that lived on the South Instead, he adapted the ideas of
were birds with differently shaped American mainland. geological renewal put forward by
beaks, adapted to different diets. James Hutton into a theory known
Darwin suggested that the as “uniformitarianism.” Earth was
In his own, bestselling account various mockingbirds might have continually being formed, altered,
of his adventure, The Voyage of the evolved from a common ancestor and reformed over immense time
Beagle, Darwin wrote, “Seeing this that had somehow crossed the periods by processes such as wave
gradation and diversity of structure Pacific from the mainland; then erosion and volcanic upheaval that
in one small, intimately related each group of birds evolved by were the same as those happening
group of birds, one might really adapting to the particular today. There was no need to invoke
fancy that from an original paucity environment on each island and disastrous interventions by God.
of birds in this archipelago, one its available food. Observing giant
species had been taken and Lyell’s ideas transformed
the way Darwin interpreted the
landscape formations, rocks, and

This giant tortoise is only found on
the Galápagos Islands, where unique
subspecies have developed on each
island. Darwin gathered evidence
here for his theory of evolution.

A CENTURY OF PROGRESS 147

The finches of the Galápagos have evolved
differently shaped beaks adapted to specific diets.

Large ground Geospiza Geospiza fortis Medium-sized
finch has a large, magnirostris ground finch has
strong beak for a smaller beak for
crushing large, crushing smaller,

woody seeds. softer seeds.

Small tree finch Warbler finch
has a short, sharp has a thin beak for
beak for grasping probing for small

insects. insects and
spearing them.
Camarhynchus Certhidea
parvulus olivacea

fossils he found on his explorations, potential to double after one evidence to support his theory of
which he now saw “through Lyell’s generation of 25 years, then double evolution. Scientists around the
eyes.” However, while he was in again in the next generation, and world sent him specimens and
South America, volume two of so on. However, food supplies data. He studied the domestication
Principles of Geology arrived. In could not expand in the same way, of animals and plants, and the
it, Lyell rejected ideas of gradual and the result was a struggle for role of selective breeding, or artificial
evolution of plants and animals, existence. Malthus’s ideas were selection, especially in pigeons. In
including Lamarck’s theories. one of the main inspirations for 1855, he started breeding varieties
Instead, he invoked the concept Darwin’s theory of evolution. of Columbia livia, or rock doves, and
of “centres of Creation” to explain they would feature prominently in
species’ diversity and distribution. The quiet years the first two chapters in On the
Although Darwin admired Lyell as Even before the Beagle had Origin of Species.
a geologist, he had to discount this returned to England, the interest
latest concept as the evidence for generated by the specimens Through his work on pigeons,
evolution mounted. Darwin had sent back had made Darwin began to understand
him a celebrity. After his return, the extent and relevance of
Another piece of the jigsaw his scientific and popular accounts variation among individuals.
was revealed in 1838 when Darwin of the voyage increased his fame. He rejected the accepted wisdom
read An Essay on the Principle However, his health deteriorated that environmental factors were
of Population by the English and gradually he withdrew from responsible for such differences,
demographer Thomas Malthus, the public eye. insisting that reproduction was
which had been published 40 years the cause, with variation somehow
earlier. Malthus described how In 1842, Darwin moved to the inherited from parents. He added
human populations can increase peace and quiet of Down House in this to the ideas of Malthus and
in an exponential way, with the Kent, where he continued to amass applied them to the natural world. ❯❯

148 CHARLES DARWIN

Much later, in his autobiography, Alfred Russel Wallace, like Darwin, nonetheless present, among the
Darwin recalled his reaction when developed his theory of evolution in the offspring within a species. For
he first read Malthus back in 1838. light of extensive field work, conducted evolution, these variations must
“Being well prepared to appreciate first in the Amazon River Basin and fulfill two criteria. One: they should
the struggle for existence…it later in the Malay Archipelago. have some effect on the struggle
at once struck me that under to survive and breed, that is, they
these circumstances favourable Worried about precedence, Darwin should help to confer reproductive
variations would tend to be consulted Charles Lyell. They success. Two: they should be
preserved, and unfavourable ones agreed to a joint presentation of inherited, or passed to offspring,
to be destroyed. The result of this Darwin’s and Wallace’s papers at where they would confer the same
would be the formation of new the Linnaean Society in London on evolutionary advantage.
species…I had at last got a theory July 1, 1858. Neither author would
by which to work.” attend in person. The audience’s Darwin describes evolution as
response was polite, with no outcry a slow and gradual process. As a
Knowing more about the role of about blasphemy. Encouraged, population of organisms adapts
variation, by 1856 Darwin the pigeon Darwin now finished his book. to a new environment, it becomes
breeder could imagine not humans Published on November 24, 1859, a new species, different from
but nature doing the choosing. From On the Origin of Species sold out
the term “artificial selection” he on its first day.
derived “natural selection.”
Darwin’s theory
Jolt into action Darwin states that species are not
On June 18, 1858, Darwin received immutable. They change, or evolve,
a short essay by a young British and the main mechanism for this
naturalist named Alfred Russel change is natural selection. The
Wallace. Wallace described a process relies on two factors.
flash of insight in which he had First, more offspring are born than
suddenly understood how evolution can survive when faced with the
occurred, and asked Darwin for challenges of climate, food supply,
his opinion. Darwin was startled competition, predators, and
to read that Wallace’s insight diseases; this leads to a struggle
replicated almost exactly the same for existence. Second, there is
ideas he himself had been working variation, sometimes tiny but
on for more than 20 years.

Charles Darwin Born in Shrewsbury, England, and on marine invertebrates,
in 1809, Darwin was originally especially barnacles, which he
destined to follow his father studied for almost 10 years. He
into medicine, but his childhood also wrote works on fertilization,
was filled with pursuits such as of orchids, insect-eating plants,
beetle collecting, and with little movement in plants, and
inclination to become a physician, variation among domesticated
he trained for the clergy. A chance animals and plants. Later in life,
appointment in 1831 placed him he tackled the origin of humans.
as expedition scientist on HMS
Beagle’s around-the-world trip. Key works

Following the voyage, Darwin 1839 The Voyage of the Beagle
was under the scientific spotlight, 1859 On the Origin of Species by
gaining fame as a perceptive Means of Natural Selection
observer, reliable experimenter, 1871 The Descent of Man, and
and talented writer. He wrote Selection in Relation to Sex
on the formation of coral reefs