theory of gases and it was not until 1908, when Perrin’s^10 experiments confirmed
Einstein’s atomistic analysis of Brownian motion that the reality of atoms was at
last accepted by such eminent holdouts as Boltzmann’s opponent Ostwald.^11
The atom has an internal structure; it is thus not “atomic” in the Greek sense and
is more than the mere restless particle of kinetic theories of gases or of Brownian
motion. This was shown by two lines of work: the study of the passage of electricity
through gases and the behavior of certain solutions. The study of the passage of
electricity through gases at low pressure was a very active field of research in the
nineteenth century and only a few of the pioneers in what we can now see as the
incipient field of subatomic physics will be mentioned here. The observation by
Pl€ucker in 1858 of a fluorescent glow near the cathode on the glass walls of a
current-carrying evacuated tube was one of the first inklings that particles might be
elicited from atoms. That these were indeed particles rather than electromagnetic
rays was indicated by Crookes in the 1870s, by showing that they could be deflected
by a magnet. Goldstein showed in 1886 the presence of particles of opposite charge
to those emitted from the cathode, and christened the latter “cathode rays”. That the
cathode rays were negative particles was proved by Perrin in 1895, when he showed
that they imparted a charge to an object on which they fell. Further evidence of the
particle nature of cathode rays came at around the same time from Thomson,^12 who
showed (1897) that they are deflected in the expected direction by an electric field.
Thomson also measured their mass-to-charge ratio and from the smallest possible
value of charge in electrochemistry calculated the mass of these particles to be
about 1/1,837 of the mass of a hydrogen atom. Lorentz later applied the name
“electron” to the particle, adopting a term that had been appropriated from the
Greek by Stoney for a unit of electric current (elektron: amber, which acquires a
charge when rubbed). Thomson has been called the discoverer of the electron.
It was perhaps Thomson who first suggested a specific structure for the atom in
terms of subatomic particles. His “plum pudding” model (ca. 1900), which placed
electrons in a sea of positive charge, like raisins in a pudding., accorded with the
then-known facts in evidently permitting electrons to be removed under the influ-
ence of an electric potential. The modern picture of the atom as a positive nucleus
with extranuclear electrons was proposed by Rutherford^13 in 1911. It arose from
(^10) Jean Perrin, born Lille, France, 1870. Ph.D. E ́cole Normal Supe ́rieure, Paris. Professor Univer-
sity of Paris. Nobel Prize in physics 1923. Died New York, 1942.
(^11) Wilhelm Friedrich Ostwald, German chemist, born Riga, Latvia, 1853. Ph.D. Dorpat, Estonia.
Professor Riga, Leipzig. A founder of physical chemistry, opponent of the atomic theory till
convinced by the work of Einstein and Perrin. Nobel Prize in chemistry 1909. Died near Leipzig,
1932.
(^12) Sir Joseph John Thomson, born near Manchester, 1856. Professor, Cambridge. Nobel Prize in
physics 1906. Knighted 1908. Died Cambridge, 1940.
(^13) Ernest Rutherford (Baron Rutherford), born near Nelson New Zealand, 1871. Studied at Cam-
bridge under J. J. Thomson. Professor McGill University (Montreal), Manchester, and Cambridge.
Nobel prize in chemistry 1908 for work on radioactivity, alpha particles, and atomic structure.
Knighted 1914. Died London, 1937.
4.2 The Development of Quantum Mechanics. The Schr€odinger Equation 93