THE REALITY OF MOLECULES 85tions for n (hence for N) and d. (Loschmidt applied his reasoning to air, for which
A was known experimentally. However, in order to estimate the densities of liquid
oxygen and nitrogen, he had to use indirect theoretical estimates.)It is not surprising that, on the whole, molecular reality met with less early
resistance in physics than it did in chemistry. As is exemplified by Loschmidt's
1866 calculation, physicists could already do things with molecules at a time when
chemists could, for most purposes, take them to be real or leave them as coding
devices. However, it became increasingly difficult in chemical circles to deny the
reality of molecules after 1874, the year in which Jacobus Henricus van 't Hoff
and Joseph Achille Le Bel independently explained the isomerism of certain
organic substances in terms of stereochemical properties of carbon compounds.
Even then skeptics did not yield at once (van 't Hoff himself was initially quite
cautious on the issue, [N2]). But by the 1880s, the power of a truly molecular
picture was widely recognized.
In order to complete this survey of topics bearing on molecular reality prior to
the time Einstein got involved, it is necessary to add two further remarks.
- The End of Indivisibility. Until the very last years of the nineteenth cen-
tury, most if not all physicists who believed in the reality of atoms shared Max-
well's view that these particles remain unbroken and unworn. 'They are. .. the
only material things which still remain in the precise condition in which they first
began to exist,' he wrote in his book Theory of Heat [M7], which contains the
finest expression of his atomic credo.* It is true that many of these same physicists
(Maxwell among them) were convinced that something had to rattle inside the
atom in order to explain atomic spectra. Therefore, while there was a need for a
picture of the atom as a body with structure, this did not mean (so it seemed) that
one could take the atom apart. However, in 1899, two years after his discovery of
the electron, Joseph John Thomson announced that the atom had been split:
'Electrification [that is, ionization] essentially involves the splitting of the atom, a
part of the mass of the atom getting free and becoming detached from the original
atom' [T3]. By that time it was becoming increasingly clear that radioactive phe-
nomena (first discovered in 1896) also had to be explained in terms of a divisible
atom. 'Atoms [of radioactive elements], indivisible from the chemical point of view,
are here divisible,' Marie Curie wrote in 1900 [C3]. She added that the expla-
nation of radioactivity in terms of the expulsion of subatomic particles 'seriously
undermines the principles of chemistry.' In 1902 Ernest Rutherford and Frederick
Soddy proposed their transformation theory, according to which radioactive bodies
contain unstable atoms, a fixed fraction of which decay per unit time. Forty years
later, a witness to this event characterized the mood of those early times: 'It must
be difficult if not impossible for the young physicist or chemist to realize how
*To Maxwell, electrolytic dissociation was not at variance with the indivisibility of atoms—but that
is another story.