THE REALITY OF MOLECULES 1O3found that the link between critical opalescence and Rayleigh scattering is quan-
titative and, once again, obtained (for the last time) new methods for measuring
Avogadro's number. As we read in Perrin's Les Atomes, these measurements were
made shortly afterward.
Smoluchowski was delighted. In a paper published in 1911, he spoke of Ein-
stein's contribution as 'a significant advance' [S9]. However, he had not quite
understood Einstein's argument. In an appendix to his 1911 paper Smoluchowski
mentioned that the blue of the sky is due to two factors: scattering off molecules
and scattering that results from density fluctuations. Einstein objected by letter
[El9]. There is one and only one cause for scattering: 'Reileigh [sic] treats a spe-
cial case of our problem, and the agreement between his final formula and my
own is no accident.' Shortly thereafter, Smoluchowski replied; 'You are completely
right' [S10J.
Smoluchowski's last contribution to this problem was experimental: he wanted
to reproduce the blue of the sky in a terrestrial experiment. Preliminary results
looked promising [Sll], and he announced that more detailed experiments were
in progress. He did not live to complete them.*
After Smoluchowski's death, Sommerfeld [S12] and Einstein [E16] wrote obit-
uaries in praise of a good man and a great scientist. Einstein called him an inge-
nious man of research and a noble and subtle human being.
Finally:
Einstein's paper on critical opalescence and the blue of the sky was written in
October 1910. It was submitted from Zurich, where he was an associate professor
at the university. It was his last major paper on classical statistical physics. In
March 1911 he moved to Prague—to become a full professor for the first time—
and began his main attack on general relativity.
Ostwald conceded in 1908. Referring to the experiments on Brownian motion
and those on the electron, he stated that their results 'entitle even the cautious
scientist to speak of an experimental proof for the atomistic constitution of space-
filled matter' [O3].
Mach died in 1916, unconvinced.**
Perrin received the Nobel prize in 1926 for his work on Brownian motion. Les
Atomes, one of the finest books on physics written in the twentieth century, con-
tains a postmortem, in the classical French style, to the struggles with the reality
of molecules:
*For references to later experimental work on critical opalescence, see, e.g., [C7]. The problems of
the modern theory of critical opalescence are reviewed in [M9].
"'Stefan Meyer recalled Mach's reaction upon being shown, in Vienna, the scintillations produced
by alpha particles: 'Now I believe in atoms' [M10]. Mach's text on optics, written after he left
Vienna, shows that this belief did not last, however [Mil].