THE REALITY OF MOLECULES 83one of them its life' [P3]. This is the battle which Ostwald joined in 1895 when
he addressed a meeting of the Deutsche Gesellschaft fur Naturforscher und Arzte:
'The proposition that all natural phenomena can ultimately be reduced to
mechanical ones cannot even be taken as a useful working hypothesis: it is simply
a mistake. This mistake is most clearly revealed by the following fact. All the
equations of mechanics have the property that they admit of sign inversion in the
temporal quantities. That is to say, theoretically perfectly mechanical processes
can develop equally well forward and backward [in time]. Thus, in a purely
mechanical world there could not be a before and an after as we have in our world:
the tree could become a shoot and a seed again, the butterfly turn back into a
caterpillar, and the old man into a child. No explanation is given by the mechan-
istic doctrine for the fact that this does not happen, nor can it be given because of
the fundamental property of the mechanical equations. The actual irreversibility
of natural phenomena thus proves the existence of processes that cannot be
described by mechanical equations; and with this the verdict on scientific materi-
alism is settled' [Ol]. It was in essence a replay of the argument given by Lo-
schmidt twenty years earlier.
Such were the utterances with which Boltzmann, also present at that meeting,
had to cope. We are fortunate to have an eye-witness report of the ensuing dis-
cussion from a young physicist who attended the conference, Arnold Sommerfeld.
'The paper on "Energetik" was given by Helm* from Dresden; behind him stood
Wilhelm Ostwald, behind both the philosophy of Ernst Mach, who was not pres-
ent. The opponent was Boltzmann, seconded by Felix Klein. Both externally and
internally, the battle between Boltzmann and Ostwald resembled the battle of the
bull with the supple fighter. However, this time the bull was victorious over the
torero in spite of the latter's artful combat. The arguments of Boltzmann carried
the day. We, the young mathematicians of that time, were all on the side of Boltz-
mann; it was entirely obvious to us that one could not possibly deduce the equa-
tions of motion for even a single mass point—let alone for a system with many
degrees of freedom—from the single energy equation ...' [SI]. As regards the
position of Ernst Mach, it was anti-atomistic but of a far more sober variety than
Ostwald's: 'It would not become physical science [said Mach] to see in its self-
created, changeable, economical tools, molecules and atoms, realities behind phe-
nomena ... the atom must remain a tool... like the function of mathematics'
[M5].
Long before these learned fin de siecle discourses took place, in fact long before
the laws of thermodynamics were formulated, theoretical attempts had begun to
estimate the dimensions of molecules. As early as 1816 Thomas Young noted that
'the diameter or distance of the particles of water is between the two thousand and
"The physicist Georg Helm was an ardent supporter of Ostwald's 'Energetik,' according to which
molecules and atoms are but mathematical fictions and energy, in its many forms, the prime physical
reality.