82 STATISTICAL PHYSICSbe, let its defects be laid bare and examined. Let them be remedied if possible, or
let the theory be rejected, and some other theory be used in its stead, if its defects
are really as irremediable and as grave as is implied by the sneers of its detractors'
[Wl].
As a final comment on chemistry in the nineteenth century, mention should be
made of another regularity bearing on the atomicity of matter and discovered in
that period. In an anonymous paper written in 1815, William Prout, a practising
physician in London with a great interest in chemistry, claimed to have shown
that the specific gravities of atomic species can be expressed as integral multiples
of a fundamental unit [PI]. In an addendum written the next year, and also pub-
lished anonymously [P2], he noted that this fundamental unit may be identified
with the specific gravity of hydrogen: 'We may almost consider the TT/OWTTJ uXi; of
the ancients to be realized in hydrogen.' Yet Prout did not consider his hypothesis
as a hint for the reality of atoms: 'The light in which I have always been accus-
tomed to consider it [the atomic theory] has been very analogous to that in which
I believe most botanists now consider the Linnean system; namely, as a conven-
tional artifice, exceedingly convenient for many purposes but which does not rep-
resent nature' [B2].
- Kinetic Theory. The insight that gases are composed of discrete particles
dates back at least to the eighteenth century. Daniel Bernoulli may have been the
first to state that gas pressure is caused by the collisions of particles with the walls
within which they are contained [B3]. The nineteenth century masters of kinetic
theory were atornists—by definition, one might say. In Clausius's paper of 1857,
'On the Kind of Motion We Call Heat' [C2], the distinction between solids, liq-
uids, and gases is related to different types of molecular motion. In 1873, Maxwell
said, 'Though in the course of ages catastrophes have occurred and may yet occur
in the heavens, though ancient systems may be dissolved and new systems evolved
out of their ruins, the molecules [i.e., atoms!] out of which these systems [the earth
and the whole solar system] are built—the foundation stones of the material uni-
verse—remain unbroken and unworn. They continue this day as they were cre-
ated—perfect in number and measure and weight ...' [M4].*
Boltzmann was less emphatic and in fact reticent at times, but he could hardly
have developed his theory of the second law had he not believed in the particulate
structure of matter. His assertion that entropy increases almost always, rather
than always, was indeed very hard to swallow for those who did not believe in
molecular reality. Planck, then an outspoken skeptic, saw this clearly when in
1883 he wrote, 'The consistent implementation of the second law [i.e., to Planck,
increase of entropy as an absolute law] ... is incompatible with the assumption
of finite atoms. One may anticipate that in the course of the further development
of the theory a battle between these two hypotheses will develop which will cost
*Faraday had reservations. In 1844 he wrote, 'The atomic doctrine ... is at best an assumption of
the truth of which we can assert nothing, whatever we may say or think of its probability' [W2].