THE REALITY OF MOLECULES 8lmore than one compound of two elements, then the ratios of the amounts of weight
of one element which bind with the same amounts of the other are simple integers.
As said, the publication of Dalton's major opus began in 1808. In 1809, Gay-
Lussac published his law of combining volumes: the proportions by volume in
which gases combine are simple integers. Gay-Lussac mentioned that his results
were in harmony with Dalton's atomic theory [Gl]. Dalton, on the other hand,
did not believe Gay-Lussac: 'His notion of measures is analogous to mine of atoms;
and if it could be proved that all elastic fluids have the same number of atoms in
the same volume, of numbers that are as 1, 2, 3, 4, etc., the two hypotheses would
be the same, except that mine is universal and his applies only to elastic fluids.
Gay-Lussac could not but see that a similar hypothesis had been entertained by
me and abandoned as untenable' [D2]. (Elastic fluids are now better known as
gases.) Also, Dalton did not accept the hypothesis put forward in 1811 by Amedeo
Avogadro, that for fixed temperature and pressure equal volumes of gases contain
equal numbers of molecules [Al].* Nor was Dalton's position one held only by a
single person for a brief time. By all accounts the high point of the Karlsruhe
congress was the address by Cannizzaro, in which it was still necessary for the
speaker to emphasize the importance of Avogadro's principle for chemical consid-
erations.** That conference did not at once succeed in bringing chemists closer
together. 'It is possible that the older men were offended by the impetuous behav-
ior and imposing manner of the younger scientists' [M2]. However, it was recalled
by Dmitri Ivanovich Mendeleev thirty years later that 'the law of Avogadro
received by means of the congress a wider development, and soon afterwards con-
quered all minds' [M3].
The law of Avogadro is the oldest of those physical-chemical laws that rest on
the explicit assumption that molecules are real things. The tardiness with which
this law came to be accepted by the chemists clearly indicates their widespread
resistance to the idea of molecular reality. For details of the atomic debate among
chemists, I refer the reader to recent excellent monographs [Bl, Nl]. Here I men-
tion only some revealing remarks by Alexander Williamson, himself a convinced
atomist. In his presidential address of 1869 to the London Chemical Society, he
said, 'It sometimes happens that chemists of high authority refer publicly to the
atomic theory as something they would be glad to dispense with, and which they
are ashamed of using. They seem to look upon it as something distinct from the
general facts of chemistry, and something which the science would gain by throw-
ing off entirely. ... On the one hand, all chemists use the atomic theory, and ...
on the other hand, a considerable number view it with mistrust, some with positive
dislike. If the theory really is as uncertain and unnecessary as they imagine it to
*The reason for Dalton's opposition was that he did not realize (as Avogadro did) that the smallest
particles of a gaseous element are not necessarily atoms but may be molecules.
**The views of this remarkable man are most easily accessible in the English translation, published
in 1961, of an article he wrote in 1858 [Cl].