86 STATISTICAL PHYSICSextremely bold [the transformation theory] was and how unacceptable to the
atomists of the time' [R3].
Thus, at the turn of the century, the classical atomists, those who believed both
in atoms and in their indivisibility, were under fire from two sides. There was a
rapidly dwindling minority of conservatives, led by the influential Ostwald and
Mach, who did not believe in atoms at all. At the same time a new breed arose,
people such as J. J. Thomson, the Curies, and Rutherford, all convinced of the
reality of atoms and all—though not always without trepidation, as in the case of
Marie Curie—aware of the fact that chemistry was not the last chapter in particle
physics. For them, the ancient speculations about atoms had become reality and
the old dream of transmutation had become inevitable.- The End of Invisibility. If there was one issue on which there was agree-
ment between physicists and chemists, atomists or not, it was that atoms, if they
exist at all, are too small to be seen. Perhaps no one expressed this view more
eloquently than van der Waals in the closing lines of his 1873 doctoral thesis,
where he expressed the hope that his work might contribute to bringing closer the
time when 'the motion of the planets and the music of the spheres will be forgotten
for a while in admiration of the delicate and artful web formed by the orbits of
those invisible atoms' [W3].
Direct images of atoms were at last produced in the 1950s with the field ion
microscope [M8]. In a broad sense of the word, particles smaller than atoms were
'seen' much earlier, of course. At the turn of the century, alpha particles were
perceived as scintillations on zinc sulfide screens, electrons as tracks in a cloud
chamber. In an 1828 paper entitled, in part, 'A Brief Account of Microscopical
Observations Made in the Months of June, July and August, 1827, on the Par-
ticles Contained in the Pollen of Plants' [B5], the botanist Robert Brown reported
seeing the random motion of various kinds of particles sufficiently fine to be sus-
pended in water. He examined fragments of pollen particles, 'dust or soot depos-
ited on all bodies in such quantity, especially in London,' particles from pulverized
rock, including a fragment from the Sphinx, and others. Today, we say that
Brown saw the action of the water molecules pushing against the suspended
objects. But that way of phrasing what we see in Brownian motion is as dependent
on theoretical analysis as is the statement that a certain cloud chamber track can
be identified as an electron.
In the case of Brownian motion, this analysis was given by Einstein, who
thereby became the first to make molecules visible. As a last preparatory step
toward Einstein's analysis, I must touch briefly on what was known about dilute
solutions in the late nineteenth century.
5b. The Pots of Pfeffer and the Laws of van 't HoffIn the mid-1880s, van 't Hoff, then professor of chemistry, mineralogy, and geol-
ogy at the University of Amsterdam, discovered in the course of his studies of