The Science Book

A CENTURY OF PROGRESS 149

I think I have found out However, the mechanism by which science until the 20th century,
(here’s presumption!) the inheritance occurred—how and when new discoveries in genetics
simple way by which species why some traits are passed on, were integrated into evolutionary
become exquisitely adapted others not—remained a mystery. theory, providing a mechanism for
Coincidentally, at the same time heredity. Darwin’s principle of
to various ends. that Darwin published his book, a natural selection remains key to
Charles Darwin monk named Gregor Mendel was understanding the process. ■
experimenting with pea plants in
Brno (in the present-day Czech This cartoon ridiculing Darwin
Republic). His work on inherited appeared in 1871, the year in which
characteristics, reported in 1865, he applied his theory of evolution to
formed the basis of genetics, but humans—something he had been
was overlooked by mainstream careful to avoid in earlier works.

its ancestors. Meanwhile, those
ancestors may remain the same,
or they may evolve in response to
their own changing environment,
or they may lose the struggle for
survival and become extinct.

Aftermath
Faced with such a thorough,
reasoned, evidence-based
exposition of evolution by natural
selection, most scientists soon
accepted Darwin’s concept of
“survival of the fittest.” Darwin’s
book was careful to avoid any
mention of humans in connection
with evolution, other than the
single sentence, “Light will be
shed on the origin of man, and
his history.” However, there were
protests from the Church, and the
clear implication that humans had
evolved from other animals was
ridiculed in many quarters.

Darwin, as ever avoiding the
limelight, remained engrossed in
his studies at Down House. As
controversy mounted, numerous
scientists sprang to his defense.
The biologist Thomas Henry Huxley
was vociferous in supporting the
theory—and arguing the case for
human descent from apes—and
dubbed himself “Darwin’s bulldog.”

FORECASTING THE

WEATHER

ROBERT FITZROY (1805–1865)



152 ROBERT FITZROY A century and a half ago, With a barometer, two or
notions of weather three thermometers, some
IN CONTEXT prediction were deemed brief instructions, and an
little more than folklore. The man attentive observation, not of
BRANCH who changed that and gave us instruments only, but the sky
Meteorology modern weather forecasting was and atmosphere, one may
British naval officer and scientist
BEFORE Captain Robert FitzRoy. utilise Meteorology.
1643 Evangelista Torricelli Robert FitzRoy
invents the barometer, which FitzRoy is better known today
measures air pressure. as the captain of the Beagle, the Naval weather pioneers
ship that carried Charles Darwin It was no coincidence that many
1805 Francis Beaufort on the voyage that led to his theory of the first breakthroughs in
develops the Beaufort scale of evolution by natural selection. weather forecasting came from
of wind force. Yet FitzRoy was a remarkable naval officers. Knowing what
scientist in his own right. weather lay ahead was crucial in
1847 Joseph Henry proposes the days of sailing ships. Missing
a telegraph link to warn the FitzRoy was just 26 when he a good wind could have huge
eastern United States of sailed from England with Darwin financial consequences—and
storms coming from the west. in 1831. Yet he had already served being caught at sea in a storm
more than a decade at sea, and could be disastrous.
AFTER had studied at the Royal Naval
1870 The US Army Signal College at Greenwich, where he Two naval officers in particular
Corps begins creating weather was the first candidate to pass the had already made significant
maps for the whole US. lieutenant’s exam with perfect contributions. One was Irish
marks. He had even commanded
1917 The Bergen School the Beagle on an earlier survey trip
of Meteorology in Norway around South America, where the
develops the notion of importance of studying the weather
weather fronts. was impressed upon him. His
ship almost met with disaster in
2001 Systems of Unified a violent wind off the coast of
Surface Analysis use powerful Patagonia after he had ignored
computers to give highly the warning signs of falling
detailed local weather. pressure on the ship’s barometer.

Robert FitzRoy Born in 1805 in Suffolk, England, resentment of the settlers.
to an aristocratic family, Robert He returned to England in
FitzRoy joined the Navy at just 1848 to command the Navy’s
12 years old. He went on to first steamship, and was
serve many years at sea as an appointed head of the British
outstanding sea captain. He Meteorological Office when it
captained the Beagle on two major was established in 1854. There
survey voyages to South America, he developed the methods that
including the around-the-world became the foundation of
voyage with Charles Darwin. scientific weather forecasting.
FitzRoy was, however, a devout
Christian who opposed Darwin’s Key works
theory of evolution. After leaving
active service in the Navy, FitzRoy 1839 Narrative of the Voyages
became governor of New Zealand, of the Beagle
where his even-handed treatment 1860 The Barometer Manual
of the Maori earned him the 1863 The Weather Book

A CENTURY OF PROGRESS 153

See also: Robert Boyle 46–49 ■ George Hadley 80 ■ Gaspard-Gustave de Coriolis 126 ■ Charles Darwin 142–49

The weather comes in
repeated patterns.

The development of each
pattern is indicated by
signs such as air pressure,

wind direction, and
cloud type.

mariner Francis Beaufort, who Before FitzRoy began his weather Since patterns are
created a standard scale showing reporting systems, mariners had repeated, their future
the wind speed or “force” linked to already observed that winds form progress can be predicted.
particular conditions at sea, and cyclonic patterns in hurricanes, and
later on land. This allowed the that wind direction could be used to Observations from
severity of storms to be recorded predict the storm’s path. multiple locations
and compared methodically for provide a “snapshot” of
the first time. The scale ranged The Meteorogical Office weather patterns over a
from 1, indicating “light air” to In 1854, FitzRoy, encouraged by
12, “hurricane.” The first time the Beaufort, was given the task of wide area.
Beaufort scale was used was by setting up the British contribution
FitzRoy on the Beagle voyage. at the Meteorological Office. From the
Thereafter it became standard But with characteristic zeal and snapshot,
in all naval ships’ logs. insight, FitzRoy went much further meteorologists
than his brief. He began to see can forecast the
Another naval weather pioneer that a system of simultaneous weather.
was American Matthew Maury. weather observations from around
He created wind and current the world could not only reveal
charts for the North Atlantic, hitherto undiscovered patterns,
which resulted in dramatic but actually be used to make
improvements for sailing times weather predictions.
and certainty. He also advocated
the creation of an international Observers already knew that
sea and land weather service, in tropical hurricanes, for example,
and led a conference in Brussels the winds blow in a circular
in 1853 that began to coordinate or “cyclonic” pattern around a
observations on conditions at central area of low air pressure or
sea from all around the world. “depression.” It was soon realized ❯❯

154 ROBERT FITZROY

FitzRoy colored his daily “synoptic”
charts in crayon. This one, made in
1863, shows a low-pressure front
bringing storms toward northern
Europe from the west. The lower right
of the chart reveals a cyclone forming.

that most of the large storms Synoptic weather at any and every point within
that blow in the mid-latitudes show FitzRoy understood that the the region covered. This was a
this cyclonic depression shape. keys to weather prediction were remarkable insight that formed
So the direction of the wind gives systematic observations of air the basis of modern forecasting.
a clue as to whether the storm is pressure, temperature, and wind
approaching or receding. speed and direction taken at set The observation figures alone
times from widely spread locations. were enough, but FitzRoy also used
In the 1850s, better records of When these observations were them to create the first modern
weather events, and the use sent instantly by telegraph to his meteorological chart, the “synoptic”
of the new electric telegraph to coordinating office in London, chart that revealed the swirling
communicate over long distances, he could build up a picture or shapes of cyclonic storms as clearly
almost instantly revealed that “synopsis” of weather conditions as satellite pictures do today.
cyclonic storms, which form over over a vast area. FitzRoy’s ideas were summed
land, move eastward. In contrast, up in his book, titled simply
hurricanes (tropical North Atlantic This synopsis gave such a The Weather Book (1863), which
storms) form over water and complete picture of the weather introduced the word “forecast” and
migrate westward. So in North conditions that it not only revealed laid out the principles of modern
America when a storm hit one current weather patterns on a forecasting for the first time.
place far inland, a telegraph could wide scale, it also enabled weather
be sent to warn places farther patterns to be tracked. FitzRoy A crucial step was to divide
east that a storm was on its way. realized that weather patterns were the British Isles into weather areas,
Observers already knew that a drop repeated. From this, it was clear to collate current weather conditions,
in air pressure on the barometer him that he could figure out how and use past weather data from
gave warning of a storm to come. weather patterns may develop over each area to help make forecasts.
The telegraph allowed such a short time in the future, from how FitzRoy recruited a network of
readings to be relayed rapidly over they have developed in the past. observers, particularly at sea and
great distances and therefore gave This provides the basis for a in ports in the British Isles. He also
warnings much further in advance. detailed forecast of the weather obtained data from France and
Spain, where the idea of constant
weather observation was catching
on. Within a few years, his network

I try, by my warnings of
probable bad weather, to avoid

the need for a life-boat.
Robert FitzRoy

A CENTURY OF PROGRESS 155

was operating so effectively that FitzRoy’s legacy Having collated and duly
he could get a daily snapshot of Faced with a barrage of ridicule considered the Irish telegrams
weather patterns right across and criticism from vested interests,
Western Europe. Patterns in the the forecasts were suspended and [or from any other weather
weather were revealed so clearly FitzRoy committed suicide in 1865. area], the first forecast for that
that he could forecast how it was When it was discovered that he had
likely to change over the next day spent his fortune on his research district is drawn…and
at least—and so produce the first at the Meteorological Office, the forthwith sent out for
national forecasts. government compensated his immediate publication.
family. But within a few years,
Daily weather forecasts pressure from mariners ensured Robert FitzRoy
Every morning, weather reports that his storm warning system was
would come to FitzRoy’s office from again in widespread use. Picking ships—all continuously feeding
scores of weather stations across up the detailed forecasts and storm information into a global
Western Europe, and within an warnings for particular shipping meteorological data bank. Powerful
hour, the synoptic picture was areas is now an essential part of number-crunching supercomputers
figured out. Instantly, forecasts every mariner’s day. churn out weather forecasts that
were despatched to The Times are, in the short-term at least,
newspaper to be published for As communications technology highly accurate, and a huge range
all to read. The first weather improved and added ever more of activities, from air travel to
forecast was published by the detail to the observational data, sports events, rely on them. ■
newspaper on August 1, 1861. the value of FitzRoy’s system came
into its own in the 20th century.
FitzRoy set up a system of
signaling cones in highly visible Modern forecasting
places at ports to warn if a storm Today, the world is dotted with
was on the way and from which a network of more than 11,000
direction. This system worked so weather stations, in addition to the
well that it saved countless lives. numerous satellites, aircraft, and

Some shipowners, however,
resented the system when their
captains began to delay setting
sail if warned of a storm. There
were also problems disseminating
the forecasts in time. It took 24
hours to distribute the newspaper,
so FitzRoy had to make forecasts for
not just one day ahead but two—
otherwise the weather would have
happened by the time people read
his forecasts. He was aware that
longer-range forecasts were far
more unreliable, and was frequently
exposed to ridicule, particularly
when The Times disassociated
itself from mistakes.

This weather station, located in
the remote mountains of Ukraine,
sends data on temperature, humidity,
and wind speed via satellite to
weather supercomputers.

156 IN CONTEXT

OMNE VIVUM BRANCH
EX VIVO— Biology
ALL LIFE
FROM LIFE BEFORE
1668 Francesco Redi
LOUIS PASTEUR (1822–1895) demonstrates that maggots
arise from flies—and not
spontaneously.

1745 John Needham boils
broth to kill microbes, and
believes that spontaneous
generation has occurred
when they grow back.

1768 Lazzaro Spallanzani
shows that microbes do not
grow in boiled broth when
air is excluded.

AFTER
1881 Robert Koch isolates
microbes that cause disease.

1953 Stanley Miller and
Harold Urey create amino
acids—essential to life—in
an experiment that simulates
origin-of-life conditions.

Modern biology teaches
that living things can
only arise from other
living things by a process of
reproduction. This may seem self-
evident today, but when the basic
principles of biology were in their
infancy, many scientists adhered to
a notion called “abiogenesis”—the
idea that life could spontaneously
generate itself. Long after Aristotle
claimed that living organisms
could emerge from decaying
matter, some even believed in
methods that purported to make
creatures from inanimate objects.
In the 17th century, for example,
Flemish physician Jan Baptista
von Helmont wrote that sweaty

A CENTURY OF PROGRESS 157

See also: Robert Hooke 54 ■ Antonie van Leeuwenhoek 56–57 ■ Thomas Henry Huxley 172–73 ■
Harold Urey and Stanley Miller 274–75

Many living organisms Some of these microbes Spoilage or infection
are microscopic, and are cause spoilage of food do not occur if microbes
or infectious disease.
suspended in the air are prevented from
around us. contaminating and

reproducing.

Microbes cannot arise by spontaneous
generation. All life comes from life.

underwear and some wheat grain 1546, Italian physician Girolamo This drawing by Francesco Redi
left in a jar in the open would Fracastoro described “seeds of shows maggots turning into flies.
spawn adult mice. Spontaneous contagion,” and came close to the His work showed not only that flies
generation had its advocates until truth of the matter. But he fell short come from maggots, but also that
well into the 19th century. In 1859, of explicitly stating that they were maggots come from flies.
however, a French microbiologist living, reproducible things, and his
named Louis Pasteur devised a theory made little impact. Instead, spontaneous generation—at least
clever experiment that disproved it. people believed that infectious in so far as creatures visible to the
In the course of his studies, he also disease was caused by “miasma”— human eye were concerned. In
proved that infections were caused or noxious air—that came from 1668, he studied the process by
by living microbes—germs. rotting matter. Without a clear idea which meat becomes riddled with
of the nature of germs as microbes, maggots. He covered one piece of
Before Pasteur, the link between no one could properly appreciate meat with parchment and left
disease or decay and organisms that the transmission of infection another exposed. Only the exposed
had been suspected but never and the propagation of life were in meat became infected with
substantiated. Until microscopes effect two sides of the same coin. maggots, because it attracted flies,
could prove otherwise, the notion which deposited their eggs on it.
that there were such things as tiny First scientific observations Redi repeated the experiment with
living entities that were invisible to In the 17th century, scientists cheesecloth—which absorbed the
the naked eye seemed fanciful. In attempted to trace the origins meat’s odor and attracted flies—
of larger creatures by studying and showed that flies’ eggs taken
In the field of experimentation, reproduction. In 1661, English from the cheesecloth could then
chance favours only the physician William Harvey (known be used to “seed” uninfected meat
prepared mind. for his discovery of the circulation with maggots. Redi argued that
Louis Pasteur of blood) dissected a pregnant deer maggots could only arise from ❯❯
in an effort to discover the origin
of a fetus, and proclaimed “Omne
vivum ex ovo”—all life from eggs.
He failed to find the deer’s egg in
question, but it was at least a hint
of things to come.

Italian physician Francesco Redi
was the first to offer experimental
evidence for the impossibility of

158 LOUIS PASTEUR

flies, rather than spontaneously. I intend to suggest that experiments can be easily
However, the significance of Redi’s no such thing as abiogenesis explained. Although heat does
experiment was not appreciated, has ever taken place in the indeed kill most microbes, some
and even Redi himself did not fully past, or ever will take place bacteria, for example, can survive
reject abiogenesis, believing that it by turning into dormant, heat-
did occur in certain circumstances. in the future. resistant spores. And most
Thomas Henry Huxley microbes, as with most life, need
Among the first makers and oxygen from the air in order to
users of the microscope for detailed had arisen spontaneously from the derive energy from their nutrition.
scientific study, Dutch scientist sterilized broth. Two decades Most importantly, however, these
Antonie van Leeuwenhoek showed later, Italian physiologist Lazzaro sorts of experiments were always
that some living things were so Spallanzani repeated Needham’s vulnerable to contamination—
small that they could not be seen experiment, but showed that the microscopic airborne microbes can
with the naked eye—and also that microbes did not grow back if easily colonize a growth medium,
the reproduction of larger creatures he removed air from the flask. even after a brief exposure to the
depended upon similar microscopic Spallanzani thought that the air atmosphere. So in fact, neither of
living entities, such as sperm. had “seeded” the broth, but his these experiments had addressed
critics proposed instead that air conclusively the question of
Yet the idea of abiogenesis was actually a “vital force” for the abiogenisis, one way or another.
was so deeply entrenched in the new generation of microbes.
minds of scientists that many still Conclusive proof
thought that these microscopic Viewed in the context of A century later, microscopes
organisms were too small to modern biology, the results of and microbiology had advanced
contain reproductive organs and Needham’s and Spallanzani’s far enough for it to became possible
must therefore arise spontaneously. to settle the matter. Louis Pasteur’s
In 1745, English naturalist John experiment demonstrated that
Needham set out to prove it. He there were microbes suspended
knew that heat could kill microbes, in air, ready to infect any exposed
so he boiled some mutton gravy surface. First, he filtered air through
in a flask—thereby killing its cotton. Then he analyzed the
microbes—and then allowed it to contaminated cotton filters
cool. After observing the broth for a and examined the trapped dust
time, he saw that the microbes had
come back. He concluded that they

Air can get in Pasteur’s swan-neck experiment
through tube proved that a sterilized broth will remain
free of microorganisms as long as they
are prevented from falling into it from the air.

Microorganisms
get trapped in
the curve

The broth is boiled to kill When the broth cools Tilting the tube allows The microorganisms
any microoganisms in it. it remains free of microorganisms back quickly multiply again.
microorganisms. into the broth.

A CENTURY OF PROGRESS 159

with a microscope. He found it It was a crushing blow to the Louis Pasteur
to be teeming with the type of last devotees of spontaneous
microbes that had been linked generation, and marked the birth Born to a poor French family
with the decay and spoilage of of a new biology solidly founded in 1822, Louis Pasteur became
food. It looked as though infection on the disciplines of cell theory, such a towering figure in the
was caused when microbes literally biochemistry, and genetics. By the world of science that, upon
fell out of the air. This was the 1880s, German physician Robert his death, he was given a full
critical information Pasteur needed Koch had shown that the disease state funeral. After training
to succeed in the next step, when anthrax was transmitted by in chemistry and medicine,
he took up a challenge laid down by infectious bacteria. his professional career
the French Academy of Sciences— included academic positions
to disprove the idea of spontaneous Nevertheless, nearly a century at the French universities
generation once and for all. after Huxley’s address, abiogenesis of Strasbourg and Lille.
would once again focus the minds
For his experiment, Pasteur of a new generation of scientists as His first research was on
boiled nutrient-rich broth—just as they began to ask questions about chemical crystals, but he is
Needham and Spallanzani had done the origin of the very first life on better known in the field of
a century before—but this time Earth. In 1953, American chemists microbiology. Pasteur showed
made a critical modification to the Stanley Miller and Harold Urey that microbes turned wine
flask. He heated the flask’s neck to sent electrical sparks through into vinegar and soured milk,
soften the glass, then drew the glass a mix of water, ammonia, methane, and developed a heat-treating
outward and downward to form a and hydrogen to simulate the process that killed them—
tube in the shape of a swan’s neck. atmospheric conditions at the dawn known as pasteurization. His
When the setup had cooled, the tube of life on Earth. Within weeks, they work on microbes helped to
was part-way directed downward so had created amino acids—the develop modern germ theory:
that microbes could not fall onto the building blocks of proteins and key the idea that some microbes
broth, even though the temperature chemical constituents of living caused infectious disease.
was now suitable for their growth cells. Miller and Urey’s experiment Later in his career, he
and there was plentiful oxygen triggered a resurgence of work developed several vaccines,
since the tube communicated with directed at showing that living and established the Institut
the outside air. The only way organisms can emerge from Pasteur devoted to the study
microbes could grow back in the nonliving matter, but this time of microbiology, which thrives
flask was spontaneously—and this scientists were equipped with to this day.
did not happen. the tools of biochemistry and an
understanding of processes that Key works
As final proof that microbes took place billions of years ago. ■
needed to contaminate the broth 1866 Studies on Wine
from the air, Pasteur repeated the I observe facts alone; 1868 Studies on Vinegar
experiment, but snapped off the I seek but the scientific 1878 Microbes: Their Roles in
swan-necked tube. The broth conditions under which Fermentation, Putrefaction,
became infected: he had finally and Contagion
disproved spontaneous generation, life manifests itself.
and had shown that all life came Louis Pasteur
from life. It was clear that microbes
could no more spontaneously
appear in a flask of broth than
mice could appear in a dirty jar.

Abiogenesis returns
In 1870, English biologist Thomas
Henry Huxley championed
Pasteur’s work in a lecture entitled
“biogenesis and abiogenesis.”

ONE OF THE SNAKES

GRABBED

ITS OWN TAIL

AUGUST KEKULÉ (1829–1896)



162 AUGUST KEKULÉ T he early years of the I spent a part of the night
19th century saw huge putting at least sketches of
IN CONTEXT developments in chemistry those musings down on paper.
that fundamentally changed the This is how the structural
BRANCH scientific view of matter. In 1803,
Chemistry John Dalton suggested that each theory came into being.
element was made of atoms that Friedrich August Kekulé
BEFORE are unique to that element, and
1852 Edward Frankland used the concept of atomic weight elements. Atoms and molecules
introduces the idea of valency to explain how elements always remained essentially theoretical
—the number of bonds an atom combine with each other in whole- concepts that nobody had seen
can form with other atoms. number proportions. Jöns Jakob directly, but they were concepts
Berzelius studied 2,000 compounds with growing explanatory power.
1858 Archibald Couper to investigate these proportions.
suggests that carbon atoms He invented the naming system we Valency
can link directly to one use today—H for hydrogen, C for In 1852, the first step toward
another, forming chains. carbon, and so on—and compiled an understanding of how atoms
a list of atomic weights for all 40 combine with each another was
AFTER elements that were then known. taken by English chemist Edward
1858 Italian chemist Stanislao He also coined the term “organic Frankland, who introduced the idea
Cannizzaro explains the chemistry” for the chemistry of of valency—which is the number of
difference between atoms living organisms—the term later atoms each atom of an element
and molecules, and publishes came to mean most chemistry can combine with. Hydrogen
atomic and molecular weights. involving carbon. In 1809, French has a valency of one; oxygen has
chemist Joseph Louis Gay-Lussac
1869 Dmitri Mendeleev lays explained how gases combine in
out the periodic table. simple proportions by volume,
and two years later the Italian
1931 Linus Pauling elucidates Amedeo Avogadro suggested that
the structure of the chemical equal volumes of gas contain equal
bond in general, and that of numbers of molecules. It was
the benzene molecule in clear that there were strict rules
particular, using the ideas governing the combination of the
of quantum mechanics.

The atoms of each element In the molecules of
can combine with other atoms in benzene, carbon atoms bond with

a set number of ways. This is each other to form rings, onto
called valency. which hydrogen atoms bond.

Carbon atoms have This structure came
a valency of four. to Kekulé in a vision
of a snake grabbing

its own tail.

A CENTURY OF PROGRESS 163

See also: Robert Boyle 46–49 ■ Joseph Black 76–77 ■ Henry Cavendish 78–79 ■ Joseph Priestley 82–83 ■
Antoine Lavoisier 84 ■ John Dalton 112–13 ■ Humphry Davy 114 ■ Linus Pauling 254–59 ■ Harry Kroto 320–21

a valency of two. Then, in 1858, O=O N=N HHH
British chemist Archibald Couper Oxygen Nitrogen HCCCH
suggested that bonds were formed
between self-linking carbon atoms, O HHH
and that molecules were chains of Propane
atoms bonded together. So water, HH
which was known to consist of two Water HHH
parts of hydrogen to one of oxygen, Cl C C C H
could be represented as H2O, or H HH
H–O–H, where “–” signifies a bond. HHH
Carbon has a valency of four, HCH HCCH 1-Chloropropane
making it tetravalent, so a carbon
atom can form four bonds, as in H HH H Cl H
methane (CH4), where the hydrogen Methane Ethane H CCCH
atoms are arranged in a tetrahedron
around the carbon. (Today, chemists HHH
think of a bond as representing a 2-Chloropropane
pair of electrons shared between
the two atoms, and the symbols H, Kekulé used the concept of valency to describe the
O, and C as representing the central bonds that are formed between atoms to make various
part of the appropriate atom.) molecules. Here, each bond is represented by a line.

Couper was working at the time hydrogen, oxygen, and chlorine) (see the diagram above). Some
at a laboratory in Paris. Meanwhile, could bond. Suddenly, organic compounds need double bonds to
in Heidelberg, Germany, August chemistry began to make sense, satisfy the valencies of the atoms:
Kekulé had come up with the same and chemists assigned structural the oxygen molecule (O2), for
idea, announcing in 1857 that formulae to all kinds of molecules. example, and the molecule of
carbon has a valency of four, and ethylene (C2H4). Ethylene reacts
early in 1858 that carbon atoms can Simple hydrocarbons such as with chlorine, and the result is
bond to one another. Publication of methane (CH4), ethane (C2H6), and not substitution but addition. The
Couper’s paper had been delayed, propane (C3H8) were now seen to be chlorine adds across the double
allowing Kekulé to publish a month chains of carbon atoms where the bond, to make 1,2 dichloroethane
before him and claim priority for spare valencies were occupied by (C2H4Cl2). Some compounds even
the idea of self-bonding carbon hydrogen atoms. Reacting such a have triple bonds, including the
atoms. Kekulé called the bonds compound with, say, chlorine (Cl2) nitrogen molecule (N2) and
between atoms “affinities,” and produced compounds in which one acetylene (C2H2), which is highly
explained his ideas in greater or more of the hydrogen atoms were reactive, and used in oxyacetylene
detail in his popular Textbook of replaced by chlorine atoms, making welding torches.
Organic Chemistry, which first compounds such as chloromethane
appeared in 1859. or chloroethane. One feature of this Benzene, however, remained
substitution was that chloropropane a puzzle. It turned out to have
Carbon compounds came in two distinct forms, either the formula C6H6, but is much
Figuring out theoretical models 1-chloropropane or 2-chloropropane, less reactive than acetylene,
based on evidence from chemical depending on whether the chlorine even though both compounds
reactions, Kekulé declared that was attached to the middle carbon have equal numbers of carbon
tetravalent carbon atoms could link atom or one of the end carbon atoms and hydrogen atoms. Devising a ❯❯
together to form what he called a
“carbon skeleton,” to which other
atoms with other valencies (such as

164 AUGUST KEKULÉ

linear structure that was not Benzene rings This image of a hexabenzocoronene
highly reactive was a real The solution to the puzzle of molecule was captured using an atomic
conundrum. There clearly had to be benzene’s structure came to Kekulé force microscope. It is 1.4 nanometers
double bonds, but how they were in 1865 in a dream. The answer in diameter and shows carbon–carbon
arranged was a mystery. was a ring of carbon atoms, a ring bonds of different lengths.
in which all six atoms were equal,
Furthermore, benzene reacts with a hydrogen atom bonded to could be on adjacent carbon atoms,
with chlorine not by addition (like each one. This meant that the on carbon atoms separated by one
ethylene) but by substitution: a chlorine in chlorobenzene could be other carbon, or at opposite ends of
chlorine atom replaces a hydrogen attached anywhere around the ring. the ring. This turned out to be the
atom. When one of benzene’s case, and the three isomers were
hydrogen atoms is substituted Further support for this theory named ortho-, meta-, and para-
by a chlorine atom, the result is came from substituting hydrogen dichlorobenzene respectively.
only a single compound C6H5Cl, twice, to make dichlorobenzene
chlorobenzene. This seemed to (C6H4Cl2). If benzene is a six- Establishing symmetry
show that all the carbon atoms membered ring with all the carbon An unsolved mystery still remained
were equivalent, since the chlorine atoms equal, there should be three over the observed symmetry of
atom might be attached to any distinct forms, or “isomers,” of this the benzene ring. To satisfy
one of them. compound—the two chlorine atoms its tetravalency, each carbon atom
should have four bonds to other
H Cl atoms. This meant that they all
H CCCH had a “spare” bond. At first,
H CCC H Cl C C C H Kekulé drew alternating single
CC and double bonds around the
H ring, but when it became
HCH apparent that the ring had to
H H be symmetrical, he suggested
H CCCH that the molecule oscillated
H CCC H Ortho-dichlorobenzine between the two structures.
H
H The electron was not discovered
Benzene C6H6 Cl C C C Cl until 1896. The idea that bonds
CC form through the sharing of
electrons was first proposed by
HCH American chemist G. N. Wilson in
H 1916. In the 1930s, Linus Pauling
then used quantum mechanics
Meta-dichlorobenzine to explain that the six spare
H electrons in the benzene ring are
not localized in double bonds, but
Cl C C C H
CC

H C Cl
H

Para-dichlorobenzine

Kekulé suggested that double and single bonds between carbon
atoms in a benzene ring alternated (left). Two chlorine atoms can
substitute for two of the hydrogen atoms in three different ways (right).

A CENTURY OF PROGRESS 165

Kekulé described the moment that he
formulated his theory of benzene rings
as a dreamlike vision, in which he saw
a snake biting its own tail as in the
ancient symbol of the ouroboros, which
is depicted here as a dragon.

are delocalized around the ring, and
shared equally between the carbon
atoms, so that the carbon-carbon
bonds are neither single nor double,
but 1.5 (see pp.254–59). It would
take these new ideas from physics
to finally solve the puzzle of the
structure of the benzene molecule.

Dream of inspiration the manner of their motion. Today with the benzene ring theory…
Kekulé’s report of his dream is I saw how frequently two smaller I turned the chair to face the
the most cited personal account ones merged into a pair; how fireplace and slipped into a
of a flash of inspiration in all of larger ones engulfed two smaller languorous state…atoms fluttered
science. It seems that he was in a ones, still larger ones bonded three before my eyes.…Long rows,
hypnagogic state—on the edge of and even four of the small ones.” frequently linked more densely;
going to sleep: that state where everything in motion, winding and
realities and imagination slide into The second occasion was in his turning like snakes. And lo, what
each another. He described it as study in Ghent in Belgium, possibly was that? One of the snakes grabbed
Halbschlaf, or half-sleep. In fact he inspired by the ancient ouroboros its own tail and the image whirled
describes two such reveries: the symbol of a snake biting its own mockingly before my eyes.” ■
first, probably in 1855, on top of a tail: “The same thing happened
bus in south London, heading for
Clapham Road. “Atoms fluttered
before my eyes. I had always seen
these tiny particles in motion, but I
had never succeeded in fathoming

August Kekulé Friedrich August Kekulé, who structure of benzene, which
called himself August, was made him the principal architect
born on September 7, 1829 in of the theory of molecular
Darmstadt, now in the German structure. In 1895, he was
state of Hesse. While at the ennobled by Kaiser Wilhelm II,
University of Giessen, he and became August Kekulé von
abandoned the study of Stradonitz. Three of the first
architecture and switched to five Nobel prizes in chemistry
chemistry after hearing the were won by his students.
lectures of Justus von Liebig.
He eventually became professor Key works
of chemistry at Bonn University.
1859 Textbook of Organic
In 1857 and the following Chemistry
years, Kekulé published a series 1887 The Chemistry of Benzene
of papers on the tetravalence of Derivatives or Aromatic
carbon, the bonding in simple Substances
organic molecules, and the

THE DEFINITELY

EXPRESSED AVERAGE

PROPORTION

OF THREE TO ONE

GREGOR MENDEL (1822–1884)



168 GREGOR MENDEL I n the history of scientific Inherited characteristics had been
understanding, one of the observed for millennia before Mendel,
IN CONTEXT greatest of all the natural but the biological mechanism
mysteries was the mechanism of that produced phenomena such
BRANCH inheritance. The fact of heredity as identical twins was unknown.
Biology had been known ever since people
noticed that family members were over many generations—and in
BEFORE recognizably similar. Practical doing so gave rise to biological
1760 German botanist implications were everywhere— diversity. But if inheritance relied
Josef Kölreuter describes from the breeding of crops and on the blending of chemical
experiments in breeding livestock in agriculture, to the principles, surely the biological
tobacco plants, but fails to knowledge that some diseases, diversity would be diluted out
explain his results correctly. such as hemophilia, could be of existence? It would be like
passed on to children. But no one mixing paints of different colors,
1842 Swiss botanist Carl von knew how it happened. and ending up with gray. The
Nägeli studies cell division adaptations and novelties upon
and describes threadlike Greek philosophers thought which Darwin’s theory rested
bodies that are later identified that there was some sort of essence would not persist.
as chromosomes. or material “principle” that was
passed from parents to offspring.
1859 Charles Darwin Parents conveyed the principle to
publishes his theory of the next generation during sexual
evolution by natural selection. intercourse; it was supposed to
have originated in the blood, and
AFTER paternal and maternal principles
1900 Botanists Hugo de Vries, blended to make a new person.
Carl Correns, and William This idea persisted for centuries—
Bateson concurrently mainly because no one came up
“rediscover” Mendel’s laws. with anything better—but when
it reached Charles Darwin, its
1910 Thomas Hunt Morgan fundamental weakness became
corroborates Mendel’s laws all too clear. Darwin’s theory of
and confirms the chromosomal evolution by natural selection
basis for heredity. proposed that species changed

Gregor Mendel Born Johann Mendel in 1822 in on animals and concentrated on
Silesia in the Austrian Empire, breeding peas. It was this work
Mendel initially trained in that led him to devise his laws
mathematics and philosophy of heredity and develop the
before entering the priesthood critical idea that inherited
as a way of furthering his characteristics are controlled
education—changing his name by discrete particles, later
to Gregor and becoming an called genes. He became abbot
Augustinian monk. He completed of the monastery in 1868 and
his studies at the University of stopped his scientific work. On
Vienna and returned to teach at his death, his scientific papers
the abbey in Brno (now in Czech were burned by his successor.
Republic). Here, Mendel developed
his interest in inheritance—and at Key work
various times studied mice, bees,
and peas. Under pressure from 1866 Experiments in Plant
the bishop, he abandoned work Hybrizidation

A CENTURY OF PROGRESS 169

See also: Jean-Baptiste Lamarck 118 ■ Charles Darwin 142–49 ■ Thomas Hunt Morgan 224–25 ■
James Watson and Francis Crick 276–83 ■ Michael Syvanen 318–19 ■ William French Anderson 322–23

Mendel’s discovery and characteristics of plants in the shape—it was possible to identify
The breakthrough in understanding next generation, and the generation dominant and recessive varieties
inheritance came nearly a century after that. He found that alternate according to these proportions.
before the chemical structure of varieties (such as purple flower
DNA was established—and and white flower) were inherited The key conclusion
less than a decade after Darwin in fixed proportions. In the first Mendel went further and tested the
published On the Origin of Species. generation, only one variety, such inheritance of two characteristics
Gregor Mendel, an Augustinian as purple flower, came through; in simultaneously—such as flower
monk in Brno, was a teacher, the second generation, this variety color and seed color. He found that
scientist, and mathematician who accounted for three-quarters offspring ended up with different
succeeded where many better- of the offspring. Mendel called this combinations of traits and—once
known naturalists had failed. It the dominant variety. He called the again—these combinations
was, perhaps, Mendel’s skills in other variety the recessive variety. occurred in fixed proportions. In
mathematics and probability In this case, white flower was the first generation, all plants
theory that proved the difference. recessive, and made up a quarter of had both dominant traits (purple
the second generation plants. For flower, yellow seed), but in the
Mendel conducted experiments each characteristic—tall/short; second generation there was a
with the common pea, Pisum seed color; flower color; and seed mixture of combinations. ❯❯
sativum. This plant varies in
several identifiable ways, such A pea’s flower may Purebred purple
as height, flower color, seed color, be white or purple. peas crossed with
and seed shape. Mendel started purebred white peas
looking at the inheritance of one produce a first generation
characteristic at a time and applied
his mathematical mind to the of peas that are
results. By breeding pea plants, all purple.
which were easily cultivated in
the monastery grounds, he could Purple is the Breeding the first
conduct a series of experiments dominant characteristic. generation of purple plants
to obtain meaningful data. with each other produces a
White is the recessive second generation with
Mendel took critical precautions characteristic.
in his work. Recognizing that both purple and white
characteristics can skip and hide in a proportion
through generations, he was careful of 3 to 1.
to start with pea plants of “pure”
stock—such as white-flowered This is explained if inheritance
plants that only produced white- is controlled by pairs of particles
flowered offspring. He crossed
pure white-flowered plants inherited from the parents.
with pure purple-flowered ones,
pure tall with pure short, and so
on. In each case, he also precisely
controlled the fertilization: using
tweezers, he transferred pollen from
unopened flower buds to stop them
from scattering indiscriminately.
He performed these breeding
experiments many times
and documented the numbers

170 GREGOR MENDEL

For example, one-sixteenth of the inherited two identical doses of Traits disappear entirely in
plants had the combination with the particle concerned. Today we the hybrids, but reappear
both recessive traits (white flower, recognize these particles as genes. unchanged in their progeny.
green seed). Mendel concluded
that the two characteristics were Genius recognized Gregor Mendel
inherited independently of one Mendel published the results of
another. In other words, inheritance his findings in a journal of natural A few months later, German
of flower color had no effect on history in 1866, but his work failed botanist Carl Correns explicitly
inheritance of seed color and vice to make an impact in the wider described Mendel’s mechanism
versa. The fact that heredity was scientific world. The esoteric for inheritance. Meanwhile, in
precisely proportional in this way nature of his title—Experiments England—spurred on after reading
led Mendel to conclude that it was in Plant Hybridization—might have the papers of de Vries and
not due to the blending of vague restricted the readership but, Correns—Cambridge biologist
chemical principles after all, but in any case, it took more than William Bateson read Mendel’s
happened because of discrete 30 years for Mendel to be properly original paper for the first time
“particles.” There were particles appreciated for what he had and immediately recognized its
controlling flower color, particles done. In 1900, Dutch botanist significance. Bateson would
for seed color, and so on. These Hugo de Vries published the results become a champion of Mendelian
particles were transferred from of plant breeding experiments ideas, and he ended up coining
parents to offspring intact. This similar to those of Mendel— the term “genetics” for this new
explained why recessive traits including a corroboration of the field of biology. Posthumously,
could hide their effects and skip three-to-one ratio. De Vries followed the Augustinian monk had at
a generation: a recessive trait up with an acknowledgment last been appreciated.
would only show through if a plant that Mendel had got there first.
By then, work of a different
Parent generation kind—in the fields of cell biology
and biochemistry—was guiding
F biologists down new avenues
1 of research. Microscopes
were replacing plant breeding
F experiments as scientists searched
2 for clues by looking right inside
cells. Nineteenth-century biologists
3:1 proportion KEY had a hunch that the key to
Particle for white heredity lay in the cell’s nucleus.
The first generation of peas (F1) bred from Particle for purple Unaware of Mendel’s work, in
“pure” white- and purple-flowered plants all 1878, German Walther Flemming
have one particle from each parent. Purple is identified the threadlike structures
dominant, so all the F1 flowers are purple. inside cell nuclei that moved
In the second generation (F2), one plant in around during cell division.
four will inherit two “white” particles and He named them chromosomes,
produce white flowers. meaning “colored body.” Within
a few years of the rediscovery

A CENTURY OF PROGRESS 171

Hugo de Vries discovered the 3:1
ratio of characteristics in experiments
with a variety of plants in the 1890s.
He would later concede that Mendel
had a claim to priority in the discovery.

of Mendel’s work, biologists had as the Law of Segregation because hundreds or thousands of genes
demonstrated that Mendel’s the alleles segregated to form on a string of DNA. Chromosome
“particles of inheritance” were real sex cells. Mendel’s second pairs separate to create sex
and that they were carried law arose when he considered cells, and the chromosome is
on chromosomes. two characteristics. The Law of then passed on whole. This
Independent Assortment suggests means that the inheritance of
Laws of inheritance refined that the relevant genes for each traits controlled by different
Mendel had established two laws trait are inherited independently. genes on the same chromosome
of inheritance. First, the fixed is not independent. Each pea
proportions of characteristics in Mendel’s choice of plant species characteristic studied by
offspring led him to conclude that was, it turns out, fortuitous. We Mendel is due to a gene on a
the particles of inheritance came now know that the characteristics separate chromosome. If they
in pairs. There was a particle pair of Pisum sativum follow the had been on the same chromosome,
for flower color, a pair for seed color, simplest pattern of inheritance. his results would have been more
and so on. Pairs were formed at Each characteristic—such as complex and harder to interpret.
fertilization because one particle flower color—is under the control of
came from each parent—and a single type of gene that comes in In the 20th century, research
separated again when the new different varieties (alleles). However, would reveal the exceptions to
generation reproduced to form many biological characteristics— Mendel’s laws. As scientists probed
its own sex cells. If the particles such as human height—are the more deeply into the behavior of
coming together were different outcome of the interactions of genes and chromosomes, they
varieties (such as those for purple many different genes. confirmed that inheritance can
and white flower), only the dominant happen in more complicated ways
particle would be expressed. Furthermore, the genes than Mendel had found. However,
Mendel studied were inherited these discoveries build on, rather
In modern terms, the different independently. Later work would than contradict, Mendel’s findings,
varieties of genes are called alleles. show that genes can sit side-by- which laid the foundation for
Mendel’s first law became known side on the same chromosome. modern genetics. ■
Each chromosome carries
I suggest…the term
Genetics, which sufficiently
indicates that our labours are
devoted to the elucidation of
the phenomena of heredity

and variation.
William Bateson

172

AN EVOLUTIONARY
LINK BETWEEN BIRDS
AND DINOSAURS

THOMAS HENRY HUXLEY (1825–1895)

IN CONTEXT I n 1859, Charles Darwin Eleven fossils of Archaeopteryx
described his theory of have been discovered. This birdlike
BRANCH evolution by natural selection. dinosaur lived in the Late Jurassic
Biology In the heated debates that followed, period, about 150 million years ago,
Thomas Henry Huxley was the in what is now southern Germany.
BEFORE most formidable champion of
1859 Charles Darwin Darwin’s ideas, earning himself could simply have been one of
publishes On the Origin the nickname “Darwin’s bulldog.” the earliest birds, rather than a
of Species, describing his More significantly, the British feathered dinosaur. But Huxley
theory of evolution. biologist did pioneering work began to study closely the anatomy
on a key tenet in the evidence of both birds and dinosaurs, and for
1860 The first Archaeopteryx for Darwin’s theories—the idea him, the evidence was compelling.
fossil, discovered in Germany, that birds and dinosaurs are
is sold to London’s Natural closely related. A transitional fossil
History Museum. Huxley made detailed comparisons
If Darwin’s theory that species between Archaeopteryx and
AFTER gradually changed into others was various other dinosaurs, and found
1875 The “Berlin specimen” true, then the fossil record should that it was very similar to the small
of Archaeopteryx, with teeth, show how species that were very dinosaurs Hypsilophodon and
is found. different had diverged from Compsognathus. The discovery,
ancestors that were very similar. In in 1875, of a more complete
1969 US paleontologist John 1860, a remarkable fossil was found Archaeopteryx fossil, this time
Ostrom’s study of microraptor in limestone in a German quarry. with dinosaur-like teeth, seemed
dinosaurs highlights new It dated from the Jurassic period, to confirm the connection.
similarities with birds. and was named Archaeopteryx
lithographica. With wings and
1996 Sinosauropteryx, the first feathers like a bird’s, yet from the
known feathered dinosaur, is time of the dinosaurs, it seemed
discovered in China. to be an example of the kind of
missing link between species that
2005 US biologist Chris Organ Darwin’s theory predicted.
shows the similarity between
the DNA of birds and that of One sample, however, was
Tyrannosaurus rex. not nearly enough to prove the
connection between birds and
dinosaurs, and Archaeopteryx

A CENTURY OF PROGRESS 173

See also: Mary Anning 116–17 ■ Charles Darwin 142–49

Detailed studies of fossils of small dinosaurs show many
features in common with birds.

Birdlike Archaeopteryx fossils have teeth, like dinosaurs.

The similarities between the anatomy of birds and dinosaurs Thomas Henry Huxley
are too great to be a coincidence.
Born in London, Huxley
There is an evolutionary link between became an apprentice doctor
birds and dinosaurs. at 13 years old. At 21, he was
a surgeon aboard a Royal
Huxley came to believe that there dinosaur with feathered legs, Navy ship assigned to chart
was an evolutionary link between Pedopenna. Also that year, a the seas around Australia
birds and dinosaurs, but he did not groundbreaking study of DNA and New Guinea. During
imagine a common ancestor would extracted from the fossilized soft the voyage, he wrote papers
ever be found. What mattered to tissue of a Tyrannosaurus rex on the marine invertebrates
him were the very clear similarities. showed that dinosaurs are he collected, and these so
Like reptiles, birds have scales— genetically more similar to birds impressed the Royal Society
feathers are simply developments than to other reptiles. ■ that he was elected a fellow
of scales—and they lay eggs. They in 1851. On his return in 1854,
also have a host of similarities in Birds are essentially similar to Huxley became a lecturer in
bone structure. Reptiles…these animals may natural history at the Royal
School of Mines.
Nevertheless, the link between be said to be merely an
dinosaurs and birds remained extremely modified and After meeting Charles
disputed for another century. Then, aberrant Reptilian type. Darwin in 1856, Huxley
in the 1960s, studies of the sleek, Thomas Henry Huxley became a strong advocate of
agile raptor Deinonychus (a relative Darwin’s theories. In a debate
of Velociraptor) began finally to on evolution held in 1860,
convince many paleontologists Huxley won the day against
of the link between birds and these Samuel Wilberforce, Bishop of
microraptors (small predatory Oxford, who argued for God’s
dinosaurs). In recent years, a host creation. Along with his work
of finds of fossils of ancient birds showing similarities between
and birdlike dinosaurs in China has birds and dinosaurs, he
strengthened the link—including gathered evidence on the
the discovery in 2005 of a small subject of human origins.

Key works

1858 The Theory of the
Vertebrate Skull
1863 Evidence as to Man’s
Place in Nature
1880 The Coming of Age
of the Origin of Species

AN APPARENT

PERIODICITY

OF PROPERTIES

DMITRI MENDELEEV (1834–1907)



176 DMITRI MENDELEEV I n 1661, Anglo-Irish physicist Humphry Davy did not distinguish
Robert Boyle defined elements between them when he first
IN CONTEXT as “certain primitive and discovered them. Similarly,
simple, or perfectly unmingled the halogen elements chlorine
BRANCH bodies; which not being made and bromine are both pungent,
Chemistry of any other bodies, or of one poisonous oxidizing agents,
another, are the ingredients of even though chlorine is a gas
BEFORE which all those called perfectly and bromine a liquid. British
1803 John Dalton introduces mixt bodies are immediately chemist John Newlands noticed
the idea of atomic weights. compounded, and into which that when the known elements
they are ultimately resolved.” were listed in order of increasing
1828 Johann Döbereiner In other words, an element cannot
attempts first classification. be broken down by chemical
means into simpler substances.
1860 Stanislao Cannizzaro In 1803, British chemist John
publishes an extensive table of Dalton introduced the idea of
atomic and molecular weights. atomic weights (now called relative
atomic masses) for these elements.
AFTER Hydrogen is the lightest element,
1913 Lothar Meyer shows the and he gave it the value 1, which
periodic relationship between we still use today.
elements by plotting atomic
weight against volume. Law of eight The first to attempt a classification
In the first half of the 19th century, of the elements was German chemist
1913 Henry Moseley redefines chemists gradually isolated more Johann Döbereiner. By 1828, he had
the periodic table using atomic elements, and it became clear that found that some elements formed
numbers—the number of certain groups of elements had groups of three with related properties.
protons in an atom’s nucleus. similar properties. For example,
sodium and potassium are silvery
1913 Niels Bohr suggests solids (alkali metals) that react
a model for the structure of violently with water, liberating
the atom. It includes shells hydrogen gas. In fact, they are
of electrons that explain the so similar that British chemist
relative reactivity of the
different groups of elements.

The elements can be arranged in The discovery of these missing
a table according to their elements suggests that the periodic
atomic weights. table reveals important features of the

structure of the atom.

Assuming a periodicity The periodic table
of properties, predictions can be can be used to
made from the gaps in a periodic
guide experiments.
table for the discovery of
missing elements.

A CENTURY OF PROGRESS 177

See also: Robert Boyle 46–49 ■ John Dalton 112–13 ■ Humphry Davy 114 ■ Marie Curie 190–95 ■
Ernest Rutherford 206–13 ■ Linus Pauling 254–59

1 Mendeleev’s periodic table was atomic number 18

1 the precursor of 1the modern table, 1 symbol 2

1H Hshown here. He left gaps in his table H He

HYDROGEN 2 where the corresponding element HYDROGEN 13 14 15 16 17 HELIUM
had not yet beenHYdDiRsOcGoEvNered, and
34 used these to predict the properties element 5 6 7 8 9 10
of the missing elements. name
2 Li Be B C N O F Ne

LITHIUM BERYLLIUM BORON CARBON NITROGEN OXYGEN FLUORINE NEON

11 12 13 14 15 16 17 18

3 Na Mg Al Si P S Cl Ar

3 4 5 6 7 8 9SODIUM 10 11 12 ALUMINUM SILICON PHOSPHORUS SULPHUR CHLORINE ARGON
MAGNESIUM
32 33 34 35 36
PERIOD 19 20 21 22 23 24 25 26 27 28 29 30 31
Ge As Se Br Kr
4 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga
GERMANIUM ARSENIC SELENIUM BROMINE KRYPTON
POTASSIUM CALCIUM SCANDIUM TITANIUM VANADIUM CHROMIUM MANGANESE IRON COBALT NICKEL COPPER ZINC GALLIUM

37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

5 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

RUBIDIUM STRONTIUM YTTRIUM ZIRCONIUM NIOBIUM MOLYBDNUM TECHNETIUM RUTHENIUM RHODIUM PALLADIUM SILVER CADMIUM INDIUM TIN ANTIMONY TELLURIUM IODINE XENON

55 56 5771 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86

6 Cs Ba La-Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

CAESIUM BARIUM LANTHANIDE HAFNIUM TANTALUM TUNGSTEN RHENIUM OSMIUM IRIDIUM PLATINUM GOLD MERCURY THALLIUM LEAD BISMUTH POLONIUM ASTATINE RADON

87 88 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118

7 Fr Ra Ac-Lr Rf Db Sg Bh Hs Mt Uun Uuu Cn Uut Uuq Uup Uuh Uus Uuo

FRANCIUM RADIUM ACTINIDE RUTHERFORDIUM DUBNIUM SEABORGIUM BOHRIUM HASSIUM MEITNERIUM UNUNNILIUM UNUNUNIUM COPERNICIUM UNUNTRIUM UNUNQUADIUM UNUNPENTIUM UNUNHEXIUM UNUNSEPTIUM UNUNOCTIUM

57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

LANTHANIUM CERIUM PRASEODYMIUM NEODYMIUM PROMETHIUM SAMARIUM EUROPIUM GADOLINIUM TERBIUM DYSPROSIUM HOLMIUM ERBIUM THULIUM YTTERBIUM LUTETIUM

89 90 91 92 93 94 95 96 97 98 99 100 101 102 103

Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

ACTINIUM THORIUM PROTACTINIUM URANIUM NEPTUNIUM PLUTONIUM AMERICIUM CURIUM BERKELIUM CALIFORNIUM EINSTEINIUM FERMIUM MENDELEVIUM NOBELIUM LAWRENCIUM

KEY Transition metals Other metals Other non-metals Noble gases
Alkali metals Rare earth metals Metalloids Halogens Radioactive rare earths
Alkali earth metals

atomic weight, similar elements The significance of Newlands’ there must be a pattern to them.
occurred every eighth place. He achievement would not be In an effort to solve the puzzle,
published his findings in 1864. recognized for more than 20 years. he made a set of 56 playing cards,
Meanwhile, French mineralogist each labeled with the name and
In the journal Chemical News Alexandre-Émile Béguyer de major properties of one element.
Newlands wrote: “Elements Chancourtois had also noticed the
belonging to the same group patterns, publishing his ideas in Mendeleev is said to have made
appear in the same horizontal 1862, but few people noticed. his breakthrough as he was about
line. Also the numbers of similar to embark on a winter journey in
elements differ by seven or Card puzzle 1868. Before setting out, he laid out
multiples of seven…This peculiar Around the same time, Dmitri his cards on the table and began
relationship I propose to call The Mendeleev was struggling with to ponder the puzzle, as though
Law of Octaves.” The patterns the same problem as he wrote his playing a game of solitare. When
in his table make sense as far book Principles of Chemistry in his coachman came to the door for
as calcium, but then go haywire. St. Petersburg, Russia. In 1863, the luggage, Mendeleev waved him
On March 1, 1865, Newlands was there were 56 known elements, and away, saying he was busy. He
ridiculed by the Chemical Society, new ones were being discovered moved the cards back and forth
who said that he might as well list at a rate of about one a year. until finally he managed to arrange
the elements in alphabetical order, Mendeleev was convinced that all 56 elements to his satisfaction,
and refused to publish his paper. with the similar groups running ❯❯

178 DMITRI MENDELEEV

vertically. The following year, Predicting new elements The six alkali metals are all soft,
Mendeleev read a paper at the In his paper, Mendeleev made a highly reactive metals. The outer layer
Russian Chemical Society stating bold prediction: “We must expect of this lump of pure sodium has reacted
that: “The elements, if arranged the discovery of many yet unknown with the oxygen in the air to give it a
according to their atomic weight, elements—for example, two coating of sodium oxide.
exhibit an apparent periodicity elements, analogous to aluminum
of properties.” He explained that and silicon, whose atomic weights predictions that are proved true.
elements with similar chemical would be between 65 and 75.” In this case, the element gallium
properties have atomic weights (atomic weight 70, forming the
that are either of nearly the same Mendeleev’s arrangement oxide Ga2O3) was discovered in
value (such as potassium, iridium, included crucial improvements over 1875; scandium (weight 45, Sc2O3)
and osmium) or that increase Newlands’ Octaves. Below boron in 1879; and germanium (weight 73,
regularly (such as potassium, and aluminum, Newlands had GeO2) in 1886. These discoveries
rubidium, and cesium). He further placed chromium, which made made Mendeleev’s reputation.
explained that the arrangement of little sense. Mendeleev reasoned
the elements into groups in the that there must exist an as-yet Mistakes in the table
order of their atomic weights undiscovered element, and Mendeleev did make some
corresponds to their valency, which predicted that one would be found mistakes. In his 1869 paper, he
is the number of bonds the atoms with an atomic weight of about 68. asserted that the atomic weight
can form with other atoms. It would form an oxide (a compound of tellurium must be incorrect: it
formed by an element with oxygen) should lie between 123 and 126,
It is the function of science to with a chemical formula of M2O3, because the atomic weight of
discover the existence of a where “M” is the symbol for iodine is 127, and iodine should
the new element. This formula clearly follow tellurium in the table,
general reign of order in nature meant that two atoms of the
and to find the causes new element would combine
governing this order. with three oxygen atoms to
Dmitri Mendeleev make the oxide. He predicted two
more elements to fill other spaces:
one with an atomic weight of about
45, forming the oxide M2O3, and the
other with an atomic weight of 72,
forming the oxide MO2.

Critics were sceptical, but
Mendeleev had made very
specific claims, and one of the
most powerful ways to support
a scientific theory is to make

The six noble gases that occur naturally (listed in group 18 of the table) are
helium, neon, argon, krypton, xenon, and radon. They have very low chemical
reactivity because they each have a full valence shell—a shell of electrons
surrounding the atom’s nucleus. Helium has just one shell containing two
electrons, while the other elements have outer shells of eight electrons.
Radioactive radon is unstable.

He Ne Ar Kr Xe
Nucleus Electron

A CENTURY OF PROGRESS 179

according to its properties. He was This came to be called the atomic Dmitri Mendeleev
wrong—the relative atomic weight number of the element, and it is
of tellurium is in fact 127.6; it is this number that determines the The youngest of at least 12
greater than that of iodine. A element’s position on the periodic children, Dmitri Mendeleev
similar anomaly occurs between table. The fact that atomic weights was born in 1834 in a village
potassium (weight 39) and argon had given a close approximation in Siberia. When his father
(weight 40), where argon clearly followed from the fact that for the went blind and lost his
precedes potassium in the table— lighter elements, the atomic weight teaching post, Mendeleev’s
but Mendeleev was not aware of is roughly (though not exactly) mother supported the family
these problems in 1869, because twice the atomic number. with a glass factory business.
argon was not discovered until When that burned down, she
1894. Argon is one of the noble Using the table took her 15-year-old son across
gases, which are colorless, odorless, The periodic table of the elements Russia to St. Petersburg to
and hardly react with other may look like just a cataloguing receive a higher education.
elements. Difficult to detect, none system—a neat way of ordering
of the noble gases were known at the elements—but it has far greater In 1862, Mendeleev
that time, so there were no spaces importance in both chemistry married Feozva Nikitichna
for them in Mendeleev’s table. Once and physics. It allows chemists Leshcheva, but in 1876 he
argon had turned up, however, to predict the properties of an became obsessed with Anna
there were several more holes to fill, element, and to try variations Ivanova Popova, and married
and by 1898, Scottish chemist in processes; for example, if a her before his divorce from
William Ramsay had isolated particular reaction does not work his first wife was final.
helium, neon, krypton, and xenon. with chromium, perhaps it works
In 1902, Mendeleev incorporated with molybdenum, the element In the 1890s, Mendeleev
the noble gases into his table as below chromium in the table. organized new standards
Group 18, and this version of the for producing vodka. He
table forms the basis of the periodic The table was also crucial in investigated the chemistry
table we use today. the search for the structure of the of oil, and helped to set up
atom. Why did the properties of Russia’s first oil refinery.
The anomaly of the “wrong” elements repeat in these patterns? In 1905, he was elected a
atomic weights was solved in 1913 Why were the Group 18 elements member of the Royal Swedish
by British physicist Henry Moseley, so unreactive, while the elements Academy of Science, who
who used X-rays to determine the in the groups on either side were recommended him for a Nobel
number of protons in the nucleus of the most reactive of all? Such Prize, but his candidacy was
each atom of a particular element. questions led directly to the picture blocked, possibly due to his
of the structure of the atom that has bigamy. The radioactive
We must expect the discovery been accepted ever since. element 101 mendelevium
of elements analogous to is named in his honor.
aluminum and silicon— Mendeleev was to some extent
lucky to have been credited for his Key work
whose atomic weight would table. Not only did he publish his
be between 65 and 75. ideas after Béguyer and Newlands, 1870 Principles of Chemistry
Dmitri Mendeleev but also German chemist Lothar
Meyer, who plotted atomic weight
against atomic volume to show
the periodic relationship between
elements, was ahead of him, too,
publishing in 1870. As so often in
science, the time had been ripe for
a particular discovery, and several
people had reached the same
conclusion independently, without
knowing about each other’s work. ■

LIGHT

AND MAGNETISM ARE

AFFECTATIONS

OF THE SAME SUBSTANCE

JAMES CLERK MAXWELL (1831–1879)



182 JAMES CLERK MAXWELL

IN CONTEXT A magnetic field can change the
polarization of light.
BRANCH
Physics This suggests that light may be an
electromagnetic wave.
BEFORE
1803 Thomas Young’s double- Assuming light to be an electromagnetic wave,
slit experiments appear to it is possible to formulate equations to describe
show that light is a wave.
mathematically the behavior of light.
1820 Hans Christian Ørsted
demonstrates a link between The discovery of long-wavelength
electricity and magnetism. radio waves (also part of the electromagnetic

1831 Michael Faraday shows spectrum) confirms the equations.
that a changing magnetic field
produces an electric field. Light and magnetism are affectations
of the same substance.
AFTER
1900 Max Planck suggests
that in some circumstances,
light can be treated as if it
were composed of tiny “wave
packets,” or quanta.

1905 Albert Einstein shows
that light quanta, today known
as photons, are real.

1940s Richard Feynman
and others develop quantum
electrodynamics (QED) to
explain the behavior of light.

T he series of differential consequences in the 20th century, Michael Faraday. Today, Faraday
equations describing and today offers hope for unifying is perhaps best known for his
the behavior of our understanding of the universe invention of the electric motor and
electromagnetic fields developed into a comprehensive “Theory of the discovery of electromagnetic
by Scottish physicist James Clerk Everything.” induction, but it was a less
Maxwell through the 1860s and celebrated discovery that provided
1870s are rightly considered one The Faraday effect Maxwell’s departure point.
of the towering achievements in Danish physicist Hans Christian
the history of physics. A truly Ørsted’s discovery, in 1820, of For two decades, Faraday
transformative discovery, they not a link between electricity and had been attempting, on and off,
only revolutionized the way that magnetism set the stage for a to find a link between light and
scientists viewed electricity, century of attempts to discover electromagnetism. Then, in
magnetism, and light, but also the links and interconnections 1845, he devised an ingenious
laid the ground rules for an entirely between seemingly unconnected experiment that answered the
new style of mathematical physics. phenomena. It also inspired question once and for all. It involved
This would have far-reaching a significant breakthrough by passing a beam of polarized light
(one in which the waves oscillate

A CENTURY OF PROGRESS 183

See also: Alessandro Volta 90–95 ■ Hans Christian Ørsted 120 ■ Michael Faraday 121 ■ Max Planck 202–05 ■
Albert Einstein 214–21 ■ Richard Feynman 272–73 ■ Sheldon Glashow 292–93

The special theory of relativity direction, discovering the link presence felt at every point in
owes its origins to Maxwell’s between electricity, magnetism, space that lies within their range
and light almost by accident. of influence, not just when certain
equations of the lines are cut. Scientists who
electromagnetic field. Maxwell’s main concern attempted to describe the physics
was to explain just how the of electromagnetism tended to fall
Albert Einstein electromagnetic forces involved into one of two schools: those who
in phenomena such as Faraday’s saw electromagnetism as some
in a single direction, easily created induction—where a moving form of “action at a distance”
by bouncing a beam of light off a magnet induces an electric similar to Newton’s model of
smooth reflecting surface) through current—were operating. Faraday gravity, and those who believed
a strong magnetic field, and testing had invented the ingenious idea that electromagnetism was
the angle of polarization on the of “lines of force," spreading in propagated through space by
other side using a special eyepiece. concentric rings around moving waves. In general, the supporters
He found that by rotating the electric currents, or emerging and of “action at a distance” hailed from
orientation of the magnetic field, reentering the poles of magnets. continental Europe and followed
he was able to affect the angle of When electrical conductors moved the theories of electrical pioneer
polarization of the light. Based on in relation to these lines, currents André-Marie Ampère (p.120), while
this discovery, Faraday argued for flowed within them. The density of the believers in waves tended
the first time that light waves were the lines of force and the speed of to be British. One clear way of
some kind of undulation in the lines relative motion both influenced the distinguishing between the two
of force by which he interpreted strength of the current. basic theories was that action
electromagnetic phenomena. at a distance would take place
But while lines of force were instantaneously, while waves
a useful aid to understanding would inevitably take some time
the phenomenon, they did not have to propagate through space. ❯❯
a physical existence—electrical
and magnetic fields make their

Theories of
electromagnetism
However, while Faraday was
a brilliant experimentalist, it
took the genius of Maxwell to put
this intuitive idea onto sound
theoretical footing. Maxwell came
to the problem from the opposite

The pattern of iron filings around
a magnet would seem to suggest the
lines of force described by Faraday. In
fact they show the direction of the force
experienced by a charge at a given
point in an electromagentic field, as
represented in Maxwell's equations.

184 JAMES CLERK MAXWELL

Maxwell’s models Magnetic field The electrical and magnetic
Maxwell began to develop his components of an electromagnetic
theory of electromagnetism in a Wavelength wave move through space while
pair of papers published in 1855 oscillating at right angles to each other
and 1856. These were an attempt magnetism could affect the and in phase, so that both elements
to model Faraday’s lines of force orientation of an electromagnetic reach their maximum amplitudes at
geometrically in terms of the flow wave as seen in the Faraday effect. the same time, and constantly
in a (hypothetical) incompressible Developing the equations reinforce each other by induction.
fluid. He had limited success and Satisfied that the essentials of
in subsequent papers tried an his theory were correct, Maxwell Electric field
alternative approach, modeling the set out in 1864 to put it on a
field as a series of particles and sound mathematical footing. Propagation
rotating vortices. By analogy, In A Dynamical Theory of the direction
Maxwell was able to demonstrate Electromagnetic Field, he described
Ampère’s circuital law, which relates light as a pair of electrical and amount of electrical or magnetic
the electric current passing through magnetic transverse waves, potential energy a point charge
a conducting loop to the magnetic oriented perpendicular to each would experience at a specific
field around it. Maxwell also other and locked in phase in such point in the electromagnetic field.
showed that in this model, changes a way that changes to the electric
in the electromagnetic field would field reinforce the magnetic field, Maxwell went on to show
propagate at a finite (if high) speed. and vice versa (the orientation of how electromagnetic waves
the electrical wave is the one that moving at the speed of light
Maxwell derived an approximate normally determines the wave’s arose naturally from the
value for the speed of propagation, overall polarization). In the last equations, apparently settling
at about 193,060 miles/s part of his paper, he laid out a the debate about the nature of
(310,700 km/s). This value was series of 20 equations that offered a electromagnetism once and for all.
so suspiciously close to the speed complete mathematical description
of light as measured in numerous of electromagnetic phenomena in He summed up his work on the
experiments that he immediately terms of electrical and magnetic subject in the 1873 Treatise on
realized that Faraday’s intuition potentials—in other words, the Electricity and Magnetism, but,
about the nature of light must be convincing as the theory was, it
correct. In the final paper of the remained unproven at the time of
series, Maxwell described how Maxwell’s death, since the short
wavelength and high frequency of
From a long view of the history light waves made their properties
of mankind…there can be impossible to measure. However,
little doubt that the most eight years later, in 1887, German
significant event of the physicist Heinrich Hertz provided
19th century will be judged the final piece of the puzzle (and
made an enormous technological
as Maxwell’s discovery of the breakthrough) when he succeeded
laws of electrodynamics. in producing a very different form
Richard Feynman of electromagnetic wave with low

A CENTURY OF PROGRESS 185

Maxwell’s equations today, but it is his set of four James Clerk Maxwell
have had a greater impact elegant equations that now bear
Maxwell’s name. Born in Edinburgh, Scotland,
on human history than in 1831, James Clerk Maxwell
any ten presidents. While Maxwell’s work settled showed genius from an early
Carl Sagan many questions about the nature age, publishing a scientific
of electricity, magnetism, and paper on geometry at 14
frequencies and long wavelengths, light, it also served to highlight years old. Educated at the
but with the same overall speed outstanding mysteries. Perhaps universities of Edinburgh
of propagation—the form of the most significant of these was and Cambridge, he became a
electromagnetism known today the nature of the medium through professor at Marischal College
as radio waves. which electromagnetic waves in Aberdeen, Scotland, at 25
moved—for surely light waves, like years old. It was there that
Heaviside weighs in all others, required such a medium? he began his work on
By the time of Hertz’s discovery, The quest to measure this so-called electromagnetism.
there had been one other important luminiferous ether was to dominate
development that finally produced physics in the late 19th century, Maxwell was interested in
Maxwell’s equations in the form leading to the development of many other scientific problems
we know today. some ingenious experiments. of the age: in 1859, he was the
The continued failure to detect first to explain the structure of
In 1884, a British electrical it created a crisis in physics that Saturn’s rings; between 1855
engineer, mathematician, and would pave the way for the twin and 1872, he did important
physicist named Oliver 20th-century revolutions of work on the theory of color
Heaviside—a self-trained genius quantum theory and relativity. ■ vision, and from 1859 to 1866
who had already patented the he developed a mathematical
coaxial cable for the efficient The Maxwell-Heaviside equations, model for the distribution of
transmission of electrical signals— although couched in the abstruse particle velocities in a gas.
devised a way of transforming the mathematical grammar of differential
potentials of Maxwell’s equations equations, actually provide a concise A shy man, Maxwell was
into vectors. These were values description of the structure and effect also fond of writing poetry
that described both the value and of electrical and magnetic fields. and remained devoutly
the direction of the force that was religious all his life. He
experienced by a charge at a given ∇ ∙ Ε = ρ died of cancer at 48.
point in an electromagnetic field. —εΟ
By describing the direction of Key works
charges across the field rather than ∇∙Β=Ο
simply its strength at individual 1861 On Physical Lines of Force
points, Heaviside reduced a dozen ∂Β 1864 A Dynamical Theory of
of the original equations to a mere ∇ × Ε = ‒ —∂t the Electromagnetic Field
four, and in doing so made them 1872 Theory of Heat
much more useful for practical ∂Ε 1873 Treatise on Electricity
applications. Heaviside’s ∇ × Β = μΟ J + μΟεΟ —∂t and Magnetism
contribution is largely forgotten

186

RAYS WERE
COMING FROM
THE TUBE

WILHELM RÖNTGEN (1845–1923)

IN CONTEXT When an electric current Fluorescent screens
is passed through a sealed near the tube also glow,
BRANCH glass tube, cathode rays even when it is covered
Physics
cause part of the tube in black cardboard.
BEFORE to glow.
1838 Michael Faraday passes
an electrical current through Invisible rays Some unknown type of
a partially evacuated glass are coming ray must have passed
tube, producing a glowing
electric arc. from the tube. through the cardboard to
make the screen glow.
1869 Cathode rays are
observed by Johann Hittorf. L ike many scientific Cathode rays
discoveries, X-rays were This arrangement of electrodes
AFTER first observed by scientists inside a sealed container is called
1896 First clinical use of studying something else—in this a discharge tube. By the 1860s,
X-rays in diagnosis, producing case, electricity. An artificially British physicist William Crookes
an image of a bone fracture. produced electric arc (a glowing had developed discharge tubes
discharge jumping between two with hardly any air in them.
1896 First clinical use of electrodes) was first observed in German physicist Johann Hittorf
X-rays in cancer treatment. 1838 by Michael Faraday. He used these tubes to measure the
passed an electrical current electricity carrying capacity of
1897 J. J. Thomson discovers through a glass tube that had charged atoms and molecules.
that cathode rays are in been partially evacuated of air. There was no glowing arc between
fact streams of electrons. The arc stretched from the negative the electrodes in Hittorf’s tubes,
X-rays are produced when electrode (the cathode) to the but the glass tubes themselves
a stream of electrons hits a positive electrode (the anode). glowed. Hittorf concluded that the
metal target.

1953 Rosalind Franklin
uses X-rays to help her to
determine the structure
of DNA.

A CENTURY OF PROGRESS 187

See also: Michael Faraday 121 ■ Ernest Rutherford 206–13 ■
James Watson and Francis Crick 276–83

“rays” must have come from the laboratory notes were burned after Wilhelm Röntgen
cathode, or negative electrode. his death, so we cannot be sure
They were named cathode rays exactly how he discovered these Wilhelm Röntgen was born
by Hittorf’s colleague Eugen “X-rays,” but he may have first in Germany, but lived in
Goldstein, but in 1897, British observed them when he noticed the Netherlands for part of
physicist J. J. Thomson showed that a screen near his discharge his childhood. He studied
that they are streams of electrons. tube was glowing even though mechanical engineering
the tube was covered in black in Zurich before becoming
Discovering X-rays cardboard. Röntgen abandoned his a lecturer in physics at
During his experiments, Hittorf original experiment and spent the Strasbourg University
noticed that photographic plates next two months investigating the in 1874, and a professor
in the same room were becoming properties of these invisible rays, two years later. He took
fogged, but he did not investigate which are still called Röntgen rays senior positions at several
this effect any further. Others in many countries. We now know universities during his career.
observed similar effects, but that X-rays are a form of short-
Wilhelm Röntgen was the first to wavelength electromagnetic Röntgen studied many
investigate their cause—finding radiation. They have a wavelength different areas of physics,
that it was a ray that could pass ranging from 0.01–10 nanometers including gases, heat transfer,
right through many opaque (billionths of a meter). In contrast, and light. However, he is best
substances. At his request, his visible light falls between the range known for his research into
of 400–700 nanometers. X-rays, and in 1901 he was
awarded the first Nobel Prize
The first X-ray image was taken by Using X-rays today in Physics for this work. He
Röntgen of his wife Anna’s hand. The Today, X-rays are produced by firing refused to limit the potential
dark circle is her wedding ring. On a stream of electrons at a metal uses of X-rays by taking out
seeing the image, Anna is said to have target. They pass through some patents, saying that his
exclaimed: “I have seen my own death.” materials better than others, and discoveries belonged to
can be used to form images of the humanity, and gave away his
insides of the body or to detect Nobel Prize money. Unlike
metals in closed containers. In many of his contemporaries,
CT (computed tomography) scans, Röntgen used lead protective
a computer combines a series of shields in his work with
X-ray images to form a 3D image radiation. He died from
of the inside of the body. an unrelated cancer at
77 years old.
X-rays can also be used to form
images of very small objects, and Key works
X-ray microscopes were developed in
the 1940s. The image resolution that 1895 On a New Kind of Rays
is possible when using light 1897 Additional Observations
microscopes is limited by the on the Properties of X-rays
wavelengths of visible light. With
their much shorter wavelengths,
X-rays can be used to form images of
much smaller objects. Diffraction of
X-rays can be used to figure out how
atoms in crystals are arranged—a
technique that proved crucial in
elucidating the structure of DNA. ■

188

SEEING INTO
THE EARTH

RICHARD DIXON OLDHAM (1858–1936)

IN CONTEXT There are different types T he shaking caused by
of seismic wave. earthquakes spreads out
BRANCH in the form of seismic
Geology P waves are not waves, which we can detect using
detected at certain distances seismographs. While working for
BEFORE the Geological Survey of India
1798 Henry Cavendish from an earthquake… between 1879 and 1903, Richard
publishes his calculations Dixon Oldham wrote a survey of an
of the density of Earth. The …therefore rocks earthquake that struck Assam in
value is greater than the inside Earth must be 1897. In it he made his greatest
density of the surface rocks, deflecting the paths contribution to plate tectonic
showing that Earth must theory. Oldham noted that the
contain denser materials. of the waves. quake had three phases of motion,
which he took to represent three
1880 British geologist John Earth’s core has different types of wave. Two of
Milne invents the modern properties that are these were “body” waves, which
seismograph. different from those traveled through Earth. The third
in Earth’s upper layers. type was a wave that traveled
1887 Britain’s Royal Society around the surface of Earth.
funds 20 earthquake
observatories worldwide. Wave effects
The body waves Oldham identified
AFTER are today known as P waves and
1909 Croatian seismologist S waves (primary and secondary—
Andrija Mohorovicic identifies the order in which they arrive at
the seismic boundary between a seismograph). P waves are
Earth’s crust and the mantle. longitudinal waves; as the wave
passes, rocks are moved backward
1926 Harold Jeffreys claims and forward in the same direction
that the core of Earth is liquid. as the waves are traveling. S waves
are transverse waves (like the
1936 Inge Lehmann argues waves on the surface of water); the
that Earth has a solid inner rocks are moved sideways to the
core and a molten outer core. direction of the wave. P waves

A CENTURY OF PROGRESS 189

See also: James Hutton 96–101 ■ Nevil Maskelyne 102–03 ■ Alfred Wegener 222–23

travel faster than S waves, and Focus of earthquake This model of
can travel through solids, liquids, an earthquake
or gases. S waves can travel only S waves P waves shows seismic
through solid materials. waves passing
Inner through Earth
Shadow zones core and the “shadow
Later, Oldham studied seismograph zones” of the
records for many earthquakes Outer primary (P) waves
around the world, and noticed that core and secondary
there was a P-wave “shadow zone” (S) waves.
extending partway around Earth
from the earthquake location. S-wave Mantle shadPo-wwazvoene
Hardly any P waves from an shadow zone
earthquake were detected in this (bRenetfr)aPctweadves
zone. Oldham knew that the speed
at which seismic waves travel the focus of the earthquake. This not completely “shadowed,” since
inside Earth depends on the indicates an Earth interior that has some P waves are detected there.
density of the rocks. He concluded very different properties than those In 1936, Danish seismologist Inge
that properties of the rocks change of the mantle. In 1926, American Lehmann interpreted these
with depth, and the resulting geophysicist Harold Jeffreys used P waves as reflections from an
changes in speed cause refraction this evidence from S waves to inner, solid core. This is the model
(the waves followed curved paths). suggest that Earth’s core is liquid, of Earth we use today: a solid inner
The shadow zone is therefore since S waves cannot pass through core surrounded by liquid, then the
caused by a sudden change in liquids. The P-wave shadow zone is mantle with crustal rocks on top.■
the properties of rocks deep
within Earth.

Today, we know that there is
a much larger shadow zone for
S waves, which extends across
most of the hemisphere opposite

Richard Dixon Oldham grounds in 1903 and returned to The seismograph,
the United Kingdom, publishing recording the unfelt motion
Born in Dublin in 1858, the his ideas about Earth’s core in
son of the superintendent of 1906. He was awarded the Lyell of distant earthquakes,
the Geological Survey of India Medal by the Geological Society enables us to see into
(GSI), Richard Dixon Oldham of London, and was made a the earth and determine
studied at the Royal School of Fellow of the Royal Society.
Mines, before joining the GSI its nature.
himself and became Key works Richard Dixon Oldham
superintendent as well.
1899 Report of the Great
The GSI’s main work Earthquake of 12th June 1897
involved mapping the rock 1900 On the Propagation
strata, but it also compiled of Earthquake Motion to
detailed reports on earthquakes Great Distances
in India, and it is for this aspect 1906 The Constitution of the
of his work that Oldham is best Interior of the Earth
known. He retired on health

RADIATION

IS AN ATOMIC

PROPERTY

OF THE ELEMENTS

MARIE CURIE (1867–1934)



192 MARIE CURIE Like many major scientific It was necessary at this
discoveries, radiation was point to find a new term
IN CONTEXT found by accident. In 1896, to define this new property
French physicist Henri Becquerel of matter manifested by the
BRANCH was investigating phosphorescence, elements of uranium and
Physics which occurs when light falls on a thorium. I proposed the
substance that then emits light of
BEFORE a different color. Becquerel wanted word radioactivity.
1895 Wilhelm Röntgen to know whether phosphorescent Marie Curie
investigates the properties minerals also emitted X-rays, which
of X-rays. had been discovered by Wilhelm Rays produced by atoms
Röntgen a year earlier. To find out, Following Becquerel’s discovery,
1896 Henri Becquerel he placed one of these minerals on his Polish doctoral student, Marie
discovers that uranium salts top of a photographic plate that was Curie, decided to investigate
emit penetrating radiation. wrapped in thick black paper and these new “rays.” Using an
exposed both to the Sun. The electrometer—a device for
1897 J. J. Thomson discovers experiment worked—the plate measuring electrical currents—she
the electron while exploring the darkened; the mineral appeared found that air around a sample of a
properties of cathode rays. to have emitted X-rays. Becquerel uranium-containing mineral was
also showed that metals would conducting electricity. The level
AFTER block the “rays” that caused the of electrical activity depended
1904 Thomson proposes plate to darken. The next day was only on the amount of uranium
the “plum pudding” model cloudy so he could not repeat the present, not on the total mass of the
of the atom. experiment. He left the mineral on mineral (which included elements
a photographic plate in a drawer,
1911 Ernest Rutherford but the plate still darkened, even
and Ernest Marsden without the sunshine. He realized
propose the “nuclear model” that the mineral must have an
of the atom. internal source of energy, which
turned out to be the result of the
1932 British physicist breakdown of atoms of uranium in
James Chadwick discovers the mineral he was using. He had
the neutron. detected radioactivity.

Marie Curie Maria Salomea Skłodowska was the University of Paris, the first
born in Warsaw in 1867. At that woman to hold this position. She
time Poland was under Russian was also the first woman to be
rule and women were not allowed awarded a Nobel Prize, and the
into higher education. She worked first to be awarded a second
to help finance her sister’s medical Nobel. During World War I, she
studies in Paris, France, and in helped set up radiology centers.
1891 moved there herself to study She died in 1934 of anemia,
mathematics, physics, and probably caused by her long
chemistry. There, she married her exposure to radiation.
colleague, Pierre Curie, in 1895.
When her daughter was born in Key works
1897, she began teaching to help
support the family, but continued 1898 Emissions of Rays
to research with Pierre in a by Uranium and
converted shed. After Pierre’s Thorium Compounds
death, she accepted his chair at 1935 Radioactivity

A CENTURY OF PROGRESS 193

See also: Wilhelm Röntgen 186–87 ■ Ernest Rutherford 206–13 ■ J. Robert Oppenheimer 260–65

other than uranium). This led her Uranium minerals emit radiation that darkens photographic
to the belief that the radioactivity plates even when there is no light.
came from the uranium atoms
themselves, and not from any The amount of radiation from the uranium minerals depends
reactions between uranium and only on the quantity of uranium present.
other elements.
The radiation must therefore come from the uranium atoms.
Curie soon found that some
minerals that contained uranium Radiation is an atomic property
were more radioactive than of the elements.
uranium itself, and wondered
whether these minerals contained and succeeded in isolating a pure particular element always have the
another substance—one that was sample of radium in 1910. In 1911, same number of protons but may
more active than uranium. By 1898, she was awarded the Nobel Prize in have different numbers of neutrons.
she had identified thorium as Chemistry, becoming the first Atoms with different numbers of
another radioactive element. She person to win or share in two prizes. neutrons are called isotopes of the
rushed to present her findings in element. For example, an atom of
a paper to the Académie des New model of the atom uranium always has 92 protons in
Sciences, but the discovery of The Curies’ discovery of radiation its nucleus, but may have between
thorium’s radioactive properties paved the way for the two New 140 and 146 neutrons. These ❯❯
had already been published. Zealand-born physicists Ernest
Rutherford and Ernest Marsden to Marie and Pierre Curie did not have
Science double formulate their new model of the a dedicated laboratory. Most of their
Curie and her husband Pierre atom in 1911, but it was not until work was done in a leaking shed next
worked together to discover the 1932 that English physicist James to the University of Paris’s School of
additional radioactive elements Chadwick discovered neutrons and Physics and Chemistry.
responsible for the high activity the process of radiation could be
of the uranium-rich minerals fully explained. Neutrons and
pitchblende and chalcolite. By the positively charged protons are
end of 1898 they had announced subatomic particles that make up
the discovery of two new elements, the nucleus of an atom, which also
which they called polonium (after has negatively charged electrons
her native country, Poland) and buzzing around it. The protons and
radium. They attempted to prove neutrons contribute almost all the
their discoveries by obtaining pure mass of the atom. Atoms of a
samples of the two elements, but it
was not until 1902 that they obtained
0.003 oz (0.1 g) of radium chloride
from a metric ton of pitchblende.

During this time, the Curies
published dozens of scientific
papers, including one outlining their
discovery that radium could help to
destroy tumors. They did not patent
these discoveries, but in 1903, they
were jointly awarded the Nobel Prize
in Physics, along with Becquerel.
Marie continued her scientific work
after her husband’s death in 1906,

194 MARIE CURIE

Alpha decay Gamma decay larger one. Fusion also releases
energy, but the great temperatures
240 Pu 236 U and pressures required to start the
94 92 process explain why scientists
have only achieved fusion in the
4 He form of nuclear weapons. So far,
2 attempts to use nuclear fusion to
generate electricity consume more
Alpha particle energy than is released.

Beta decay Half-life
As a radioactive material decays,
22 Na 22 Ne the atoms of the radioactive
11 10 element change to other elements,
and so the number of unstable
e+ Electron neutrino atoms reduces with time. The
Beta+particle (positron) fewer unstable atoms there are, the
less radioactivity will be produced.
Radioactive decay can happen in three ways. Plutonium-240 (top left) The reduction in activity of a
decays to make uranium and an alpha particle. This is an example of radioactive isotope is measured by
alpha decay. During beta decay, sodium-22 decays to make neon, a beta its half-life. This is the time it takes
particle (in this case a positron), and a neutrino. With gamma decay, a for the activity to halve, which
high-energy nucleus gives off gamma radiation but no particles. is the same as saying the time for
the number of unstable atoms in
isotopes are named after the when a proton turns into a neutron a sample to halve. For example, the
total number of protons and or vice versa. Alpha and beta decay isotope technetium-99m is widely
neutrons, so the most common both change the number of protons used in medicine, and has a half-
isotope of uranium, with 146 in the nucleus of the decaying life of 6 hours. This means that 6
neutrons, is written as atom so that it becomes an atom hours after a dose is injected into a
uranium-238 (i.e. 92 + 146). of a different element. Gamma rays patient, the activity will be half of
are a form of high-energy short- its original level; 12 hours after
Many heavy elements, such wave electromagnetic radiation injection, the activity will be one
as uranium, have nuclei that and do not change the nature of quarter of the original level, and so
are unstable, and this leads to the element. on. By contrast, uranium-235 has a
spontaneous radioactive decay. half-life of over 700 million years.
Rutherford named the emissions Radioactive decay is different
from radioactive elements alpha, from the fission process that takes Radioactive dating
beta, and gamma rays. The nucleus place inside nuclear reactors, and This idea of half-life can be used
becomes more stable by emitting the fusion process that powers the to date minerals or other materials.
an alpha particle, a beta particle, Sun. In fission, unstable nuclei such Many different radioactive elements
or gamma radiation. An alpha as uranium-235 are bombarded with known half-lives can be used
particle consists of two protons with neutrons and break up to form to do this, but one of the best
and two neutrons. Beta particles much smaller atoms, releasing known is carbon. The most
can be electrons or their opposites, energy in the process. In fusion, common isotope of carbon is
positrons, emitted from the nucleus two small nuclei combine to form a carbon-12, with 6 protons and 6
neutrons in each atom. Carbon-12
makes up 99 percent of the carbon
found on Earth, and has a stable
nucleus. A tiny proportion of the
carbon is carbon-14, which has
two extra neutrons. This unstable

A CENTURY OF PROGRESS 195

isotope has a half-life of 5,730 years. The Curie laboratory… produce radon gas (a radioactive
Carbon-14 is constantly being was a cross between a stable gas produced when radium
produced in the upper atmosphere and a potato-cellar, and, if I decays). This was sealed into glass
as nitrogen atoms are bombarded had not seen the worktable tubes and inserted into patients’
with cosmic rays. This means with the chemical apparatus, bodies to kill diseased tissue.
there is a relatively constant ratio It was seen as a wonder cure,
of carbon-12 to carbon-14 in the I would have thought it and even marketed in beauty
atmosphere. Since photosynthesizing a practical joke. treatments to help firm up aging
plants take in carbon dioxide from skin. It was only later that the
the atmosphere, and our food Wilhelm Ostwald importance of using materials with
consists of plants (or animals that a short half-life was recognized.
have eaten plants), there is also a with other dating methods such
relatively constant proportion in as tree rings, and the corrections Radioactive isotopes are also
plants and animals while they are applied to objects of similar age. widely used in medical imaging to
alive, even though the carbon-14 diagnose disease, and in treatment
is constantly decaying. When an A wonder treatment of cancer. Gamma rays are used to
organism dies, no more carbon-14 Curie realized that radioactivity sterilize surgical instruments, and
is taken into its body, while the had medicinal uses. During World even food, to increase its shelf life.
carbon-14 already there continues War I, she used the small amount Gamma ray emitters can be used
to decay. By measuring the ratio of of radium she had extracted to for the internal inspection of metal
carbon-12 to carbon-14 in the body, objects, to detect cracks, or to
scientists can figure out how long inspect the contents of cargo
ago the organism died. containers to identify contraband. ■

This radiometric method is used The erection of Ale’s stones in
to date wood, charcoal, bone, and Sweden was dated to 600 CE by the
shells. There are natural variations radiometric dating of wooden tools
in the ratios of the carbon isotopes, found at the site. The actual stones are
but dates can be cross-checked hundreds of millions of years older.

196

A CONTAGIOUS
LIVING FLUID

MARTINUS BEIJERINCK (1851–1931)

IN CONTEXT Tobacco mosaic disease shows features of an infection, but…

BRANCH …filters that catch bacteria do not catch and remove
Biology the contagion, so it cannot be bacteria.

BEFORE Also, unlike bacteria, the infectious agent grows only
1870s and 80s Robert Koch in a living host, not in laboratory gels or broths.
and others identify bacteria as
the cause of diseases such So the causative agent must be different and
as tuberculosis and cholera. even smaller, deserving a new name—virus.

1886 German plant biologist T hese days, the word “virus” and medicine. It was suggested
Adolf Mayer shows tobacco is all too familiar as a in 1898 by Dutch microbiologist
mosaic disease can be medical term, and many Martinus Beijerinck for a new
transferred between plants. people understand the idea that category of contagious disease-
viruses are just about the smallest causing agents. Beijerinck had
1892 Dmitri Ivanovsky of the harmful agents, or germs, a special interest in plants and a
demonstrates that tobacco that cause infections in humans, skilled talent for microscopy. He
plant sap passing through the other animals, plants, and fungi. experimented with tobacco plants
finest unglazed porcelain filters that were suffering from mosaic
still carries infection. Yet at the end of the 19th disease, a discoloring mottled
century, the term virus was only effect on the leaves that was costly
AFTER just making its way into science
1903 Ivanovsky reports
light-microscope “crystal
inclusions” in infected host
cells, but suspects they are
very small bacteria.

1935 US biochemist Wendell
Stanley studies the structure
of the tobacco mosaic virus,
and realizes that viruses are
large chemical molecules.

A CENTURY OF PROGRESS 197

See also: Friedrich Wöhler 124–25 ■ Louis Pasteur 156–59 ■ Lynn Margulis 300–01 ■ Craig Venter 324–25

for the tobacco industry. His results that could pass on disease. In 1892, microbiological techniques,
led him to apply the term virus— Russian botanist Dmitri Ivanovsky Beijerinck figured out that they
already in occasional use for performed tests on tobacco mosaic really did exist. He insisted that
substances that were toxic or disease, showing that its infection they caused disease, propelling
poisonous—to the contagious agent passed through the filters. microbiology and medical science
agents that caused the disease. He established that the agent in into a new era. It would not be
this case could not be bacteria, until 1939, with the aid of electron
At the time, most of Beijerinck’s but did not investigate further to microscopes, that tobacco mosaic
contemporaries in science and discover what the agent might be. virus became the first virus to have
medicine were still grappling with its photograph taken. ■
understanding bacteria. Louis Beijerinck repeated Ivanovsky’s
Pasteur and German physician experiment. He, too, established This electron micrograph image
Robert Koch had first isolated and that even after juice pressed from shows particles of the tobacco mosaic
identified them as disease-causing the leaves was filtered, tobacco virus at 160,000x magnification.
in the 1870s, and more were being mosaic disease was still present. The particles have been stained
discovered constantly. Indeed, at first he thought that the to enhance their visibility.
cause was the fluid itself, which he
A common method of testing called contagium vivum fluidium
for bacteria at the time was to pass (contagious living fluid). He further
liquid containing the suspected demonstrated that the contagion
contagions through various sets of carried in the fluid could not be
filters. One of the best known was grown in laboratory nutrient gels
the Chamberland filter, invented in or broths, nor in any host organism.
1884 by Pasteur’s colleague Charles It had to infect its own specific
Chamberland. It used minute pores living host in order to multiply
in unglazed porcelain to capture and spread the disease.
particles as small as bacteria.
Even though viruses could not
Too small to filter be seen by light microscopes of
Several researchers had suspected the time, grown with the usual
that there was a class of infectious laboratory culture methods, or
agents even tinier than bacteria detected by any of the standard

Martinus Beijerinck Something of a recluse, Martinus as working on plant galls,
Beijerinck spent many solitary fermentation by yeasts and
hours experimenting in the other microbes, the nutrition of
laboratory. He was born in microbes, and sulfur bacteria.
Amsterdam in 1851, and studied By the end of his life, he was
chemistry and biology in Delft, internationally recognized. The
graduating in 1872 from Leiden Beijerinck Virology Prizes, set up
University. Focusing on soil and in 1965, are awarded every two
plant microbiology at Delft, he years in the field of virology.
performed his famous filtering
experiments on the tobacco Key works
mosaic virus in the 1890s. He
also studied how plants capture 1895 On Sulphate Reduction by
nitrogen from the air and Spirillum desulfuricans
incorporate it into their tissues— 1898 Concerning a contagium
a kind of natural fertilizer system vivum fluidium as a Cause of the
that enriches the soil—as well Spot-disease of Tobacco Leaves

A PARA
SHIFT

1900–1945