Review
Endeavour
Vol.31 No.2
Jean Perrin and the triumph of the
atomic doctrine
Gary Patterson
Carnegie Mellon University, Pittsburgh, PA 15213, USA
One of the central dogmas of modern science is that the
world around us can be understood in terms of microscopic chemical entities known as atoms. It may come as
a surprise that this notion has only been widely acknowledged since the 1910s. The French physicist Jean Perrin
had a hand in many of the key developments that led to
the emergence of the atomic doctrine. His life story
relates how new technologies were used to ‘see’ these
invisible particles of philosophy and how scientists were
able to determine their size and composition. The indivisible atoms of the ancients were replaced by the highly
structured elements of chemistry.
Hotel Metropole, Brussels
Most of the guests at the Hotel Metropole went happily
about their business. Little did they know that behind
closed doors, many of Europe’s greatest physicists and
chemists had gathered in an effort to synthesize the latest
developments in atomic physics. The Belgian industrialist
Ernest Solvay, whose vision the meeting had been, was
sure that good things would happen.
J.J. Thomson and Wilhelm Roëntgen had just uncovered new types of high-energy particles. Max Planck, Niels
Bohr and Albert Einstein were able to explain the actual
phenomena observed in the interaction of light and matter
in terms of discrete energy levels. Henri Becquerel and
Pierre and Marie Curie had discovered radioactivity. Was
this the beginning of a new era in science? Or was it a
passing fancy? The Dutch physicist Hendrik A. Lorentz
acted as chairman of a group of over 23 of Europe’s scientific elite as they gathered to discuss Radiation and the
Quanta – they were about to find out.
One of the attendees at the so-called Solvay Congress in
1911 was the French physicist Jean Perrin. At his birth,
some 40 years ago in 1870, science had been in danger of
devolving into a sterile game played by worn-out personalities [1]. French science, in particular, was under the heel
of the chemist Marcellin Berthelot and dominated by the
ideology of physicist and philosopher Pierre Duhem [2,3].
Such players wished to restrict the language of science to
objects large enough to be observed in the laboratory. No
occult entities were allowed to defile the Temple of Science.
Both Duhem and Berthelot worshipped at the altar of
Comte and the Positivists.
Many of the key concepts on which the modern practice
of chemistry is based had already been worked out: most
Corresponding author: Patterson, G. (gp9a@andrew.cmu.edu).
Available online 28 June 2007.
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chemists accepted the periodic table as a key organizing
principle [4]; although nobody had managed to work out
the atomic weight of each element, the significance of this
property was not in question. The New Chemical Philosophy of Dalton was widely used to rationalize the observed
compounds and their reactions, but even luminaries such
as Kekulé and van’t Hoff were careful to couch their
remarks in terms of imaginary atoms [5]. They were cowed
by skeptics such as Mach and Kolbe.
There were those who did not tow the Positivist line. In
Paris, Adolph Wurtz (1817–1884) argued passionately that
atoms and molecules were really there to be observed in the
laboratory as physical objects [6,7]. Then there were Clausius, Maxwell and Boltzmann, whose work on the kinetic
theory of gases implied the presence of particles of just
nanometers in size [8]. And champions of the atomic theory,
such as Lords Kelvin and Rayleigh, flourished in England.
But as long as no one had actually ‘seen’ an atom, skeptics
like Duhem were able to maintain that such entities were
only ‘models, dear to physicists of the English school’ [9,10].
Perrin in Paris
Jean Perrin was born in the provinces, but his ability was
recognized early and he was sent to Paris to complete his
baccalaureate. His teacher of mathematics, Emile Lacour,
prepared him for entrance to the Ecole Normale Superieure, the leading technical school in France. Settling there
in 1891, he had the good fortune to study under the
physicist and mathematician Marcel Brillouin. The time
had come for the battle of the atom.
The classical ‘atomos’ of the Greeks were ‘indivisibles’
with no internal structure, but in some cases were imagined as different shapes corresponding to the five perfect
solids [11,12]. Jean Perrin wanted to know the properties
of the actual atoms and initiated both an intellectual and
an experimental program to measure them [13]. ‘Thus we
know how to divide an atom into two parts: but the two
pieces are not of comparable size, and one of them, the
corpuscle, is very small by comparison to the atom’, he
wrote in 1901 [1,13]. It was clear that real atoms were
characterized by internal structure.
Science can often be organized around the key special
instruments that make possible unique observations. In this
case, the invention of the Crooke’s tube, a special vacuum
tube with a collection of high-voltage plates and holes, made
it possible to begin exploring the composition of individual
atoms (Figure 1). The high voltage ripped individual electrons (corpuscles) from the gaseous atoms in the tube,
producing a plasma of excited atoms and electrons.
0160-9327/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.endeavour.2007.05.003
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Vol.31 No.2
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Figure 1. The Crooke’s tube and the screen images it produced. (a) The high voltage generates ‘cathode rays’ or electrons and positively ionized atoms, which are
accelerated toward the screen and deflected by the magnetic field. (b) Ionized atoms of different charge-to-mass ratio have different trajectories, suggesting the presence of
discrete atoms and molecules. With permission from Mark Bier, Carnegie Mellon University.
These electrons were originally called ‘cathode rays’.
Perrin showed that they were characterized by a single
negative charge consistent with the principle of electricity
[14]. J.J. Thomson measured the charge-to-mass ratio of
the cathode rays and established that they were discrete
particles of known mass and charge [15]. When the electrons were accelerated in a field and impacted a metal
plate, the highly excited atoms of the metal gave off
Roentgen rays or X-rays. These particles were shown to
be electrically neutral and to have no mass [14].
The Crooke’s tube also allowed another remarkable
observation. The interaction of the dilute gas with the
cathode rays produced positively ionized particles, which
were accelerated toward the cathode. While the ‘cathode
rays’ were all identical, the positive particles were of
differing mass and charge [16,17]. There was, it seemed,
more to chemical atoms than the ancients had imagined.
In addition, there was optical spectroscopy, an experimental technique that had a huge impact on the understanding of atoms. The different colors in a sample of light
could be separated with a set of glass prisms and recorded
with a photographic plate. The visible light emission spectrum of pure atoms was obtained by using Geissler tubes: a
high vacuum tube with an anode and a cathode was filled
with the gas sample and high voltage was applied, producing a glowing plasma. The spectra were observed to be a
collection of discrete bands (or lines) of color, and every
element gave a different line spectrum. While it would be
many years before anyone could make full sense of these
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data, there was further evidence that atoms were not
simply solid balls [17].
One of Perrin’s great passions was the size of atoms. In his
classic monograph, Les Atomes, he stated that ‘The same
ardent and disinterested curiosity that has led us to weigh
the stars and map out their courses urges us towards the
infinitely small as towards the infinitely large’ [16]. The
actual limit to material division, he realized, could be
determined if he could establish the value of the Avogadro
number, a constant named after the early nineteenth-century Italian scientist Amadeo Avogadro, the first to have
realized that the volume of a gas is proportional to the
number of its atoms or molecules. Perrin approached its
value – the number of atoms needed to make up the atomic
weight of an element – from several different angles, and the
agreement between his estimates of the Avogadro number
strengthened the idea that macroscopic matter was made up
of discrete atoms of measurable volume.
Perrin also tackled the concern that no one had actually
‘seen’ an atom by looking at the Brownian motion of
particles in solution. The invention of the ultramicroscope
in 1903 by Siedentopf and Zsigmondy allowed the location
of the particles to be measured as a function of time [18]
(Figure 2). The agreement between the observed Brownian
motion and the theory presented by Einstein [19] led
Perrin to conclude that ‘The laws of perfect gases are thus
applicable in all their details to emulsions. This fact provides us with a solid experimental foundation upon which
to base the molecular theories’ [16].
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Endeavour Vol.31 No.2
Figure 2. The trajectory of a gamboge particle in colloidal solution. The location of
the particle was measured with an ultramicroscope at fixed time intervals. From
J. Perrin, Les Atomes, Paris, 1913.
Thermodynamics and the unity of science
Perrin believed passionately in the unity of science – that
there were common scientific laws that could be used to
understand all natural phenomena at all levels of organization. According to this belief, a microscopic theory
leading to macroscopic predictions that did not correspond with reality simply could not be true. Classical
calculations of the heat capacities of gases and monatomic solids were not in agreement with experimental
measurements. Was the atomic theory wrong, or was
there a need for a new kind of mechanics? Perrin concluded
that there was a need for a new physics [16]. Ultimately it
was the prediction of macroscopic thermodynamic
phenomena from the microscopic structure and dynamics
of matter that really resulted in the triumph of the atomic
doctrine.
Perrin was willing to consider new ideas to aid in the
understanding of new observations, and he kept abreast of
the new ideas being proposed by Planck, Bohr and Einstein. Application of their quantum theory to the rotational
heat capacity of atoms revealed that the moment of inertia
of atoms must be very small. And this implied that most of
the atom’s mass occupied a very small volume [16]. Rutherford’s scattering experiments, in which he fired a beam of
alpha particles at a thin layer of gold leaf, supported
Perrin’s assertions. The vast majority of alpha particles
passed clean through the metal, undeflected by the concentrated nucleus [20].
With all the new ideas and observations that had been
reported since 1897, it was time for scientists to come
together and talk about the current state of atomic theory.
Jean Perrin was one of the chosen few invited to the Solvay
Congress of 1911 (Figure 3). They discussed the need for a
new mechanics, the intimate properties of matter, the
nature of radiation, the ‘ether’ and the new quantum
approaches of Einstein and Planck. And one of the highlights of the Congress was the paper by Perrin, ‘Les preuves de la realite moleculaires’. The existence of chemical
atoms and molecules was no longer an open question.
Electrons and atoms
The concentrated nature of the nucleus suggested that it
was the electrons that determined the chemical size of an
atom, and attention now focused on what they were up to.
The classical planetary model of the atom had the electrons
whizzing around the nucleus. But since stable atoms did
not emit the light predicted for an accelerating electric
charge, this model violated the laws of electrodynamics. A
quantum picture of the atom was in order.
One vision emerged from a doctoral thesis defended
before Perrin in 1924 [21]. In this, the candidate – Louis
de Broglie – proposed the ‘crazy’ theory of electron waves,
the idea that matter, at the microscopic level, was characterized by wavelike properties. Rather than rejecting this
idea, as the thesis committee recommended, Perrin invited
Paul Langevin to join the committee and the thesis was
Figure 3. The attendees at the first Solvay Congress in 1911. Jean Perrin had direct interactions with most of them.
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approved. In 1945, it was de Broglie who delivered the
eulogy to Perrin before the French Academy of Sciences,
praising him for his ‘fecund imagination, his admirable
talent as an experimentalist, and for his persistence in
surmounting all the obstacles that were needed to confirm
the atomic hypothesis’. It was testimony to the long-term
friendship they developed [22].
One of the most cherished notions of classical ‘atoms’
was their immutability [11]. But direct observation of
actual atoms revealed that they could be played like an
electronic fiddle. An even more disquieting observation
was that some atoms ‘spontaneously’ emitted particles
resulting in new chemical elements. The speculative world
of the classical atomists was replaced with the highly
counterintuitive world of actual atoms.
Conclusions
The triumph of the atomic doctrine triggered a cascade of
Nobel laureates. In physics, prizes went to the Curies and
Becquerel (1903) for the discovery of radioactivity; Lenard
(1905) for his work on cathode rays; Thomson (1906) for his
studies of the conductivity of gases; Barkla (1917) for the
discovery of the characteristic X-rays of the elements;
Planck (1918) for the concept of quanta; Stark (1919) for
the discovery and explanation of the electrically induced
fine structure in atomic spectra; Einstein (1921) for his
explanation of the photoelectric effect; Bohr (1922) for his
studies of the electronic structure and emission spectra of
atoms; Millikan (1923) for his studies of the magnitude of
the elementary charge; Siegbahn (1924) for the application
of X-ray spectroscopy to atoms; Franck and Hertz (1925) for
the field of electron scattering from atoms; Perrin (1926) for
his demonstration of the discontinuous structure of matter;
and de Broglie (1929) for the wave nature of electrons. In
chemistry, prizes went to Ramsay (1904) for his discovery
of the noble gas elements; Rutherford (1908) for his fundamental studies of radioactive emissions and their use in the
study of atoms; Pierre Curie (1911) for the discovery of
radium and polonium; and Soddy (1921) and Aston (1922)
for the discovery and explanation of atomic isotopes [23].
The philosophical intuition that macroscopic matter
could be understood in terms of microscopic atoms has
evolved into the experimental elaboration of the actual
elements of chemistry. As Mary Jo Nye revealed in her
biography and Bernard Pullman outlined in his history of
the atom, Perrin was a witness to most of the discoveries
associated with this exciting project [1,5].
The unity of science was established for the future by
including all the observations and concepts needed to
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53
understand the world, wherever they were developed.
Perrin was an exemplar of the universal scientist who
sought truth wherever he could and by whatever means
available. In addition, he believed that science should be
used for the good of humankind. He was feted by Leon
Blum, the President of France, for his tireless efforts to
establish the CNRS, the French agency for the funding and
promotion of science. ‘Jean Perrin was both one of the most
elevated spirits and one of the wisest men of our time’ [22].
References
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