80 STATISTICAL PHYSICSalike in shape, weight, and every other particular' [Bl]. Note that Dalton's com-
pound atom is what we call a molecule. Great confusion reigned through most of
the nineteenth century regarding such terminology, one man's molecule being
another man's atom. The need for a common language developed, but slowly.
Fifty years later, at the first international scientific conference ever held, the 1860
Karlsruhe congress of chemists,* the steering committee still considered it neces-
sary to put at the top of the agenda of points to be discussed the question, 'Shall
a difference be made between the expressions molecule and atom, such that a
molecule be named the smallest particle of bodies which can enter into chemical
reactions and which may be compared to each other in regard to physical prop-
erties—atoms being the smallest particles of those bodies which are contained in
molecules?,' [Ml]. More interesting than the question itself is the fact that, even
in 1860, no consensus was reached.
Especially illuminating for an understanding of science in the nineteenth cen-
tury are the topics discussed by young August Kekule von Stradonitz (who by
then had already discovered that carbon atoms are tetravalent) in the course of his
opening address to the Karlsruhe conference. '[He] spoke on the difference
between the physical molecule and the chemical molecule, and the distinction
between these and the atom. The physical molecule, refers, he said, to the particle
of gas, liquid, or solid in question. The chemical molecule is the smallest particle
of a body which enters or leaves a chemical reaction. These are not indivisible.
Atoms are particles not further divisible' [Ml]. Both physics and chemistry could
have profited if more attention had been paid to the comment by Stanislao Can-
nizzaro, in the discussion following Kekule's paper, that the distinction between
physical and chemical molecules has no experimental basis and is therefore unnec-
essary. Indeed, perhaps the most remarkable fact about the nineteenth century
debates on atoms and molecules is the large extent to which chemists and physicists
spoke at cross purposes when they did not actually ignore each other. This is not
to say that there existed one common view among chemists, another among phys-
icists. Rather, in either camp there were many and often strongly diverging opin-
ions which need not be spelled out in detail here. It should suffice to give a few
illustrative examples and to note in particular the central themes. The principal
point of debate among chemists was whether atoms were real objects or only mne-
monic devices for coding chemical regularities and laws. The main issues for the
physicists centered around the kinetic theory of gases; in particular, around the
meaning of the second law of thermodynamics.
An early illustration of the dichotomies between the chemists and the physicists
is provided by Dalton's opinion about the work of Joseph Louis Gay-Lussac.
Dalton's chemistry was based on his law of multiple proportions: if there exists
"The meeting was held September 3-5, 1860. There were 127 chemists in attendance. Participants
came from Austria, Belgium, France, Germany, Great Britain, Italy, Mexico, Poland, Russia, Spain,
Sweden, and Switzerland.