24 ILLINOIS ACADEMY OF SCIENCE. 



case are recent. For a genuine experimental basis we must treat 

 the law as a corollary of the law of combining weights. We all 

 remember that Dalton was not successful in selecting true atomic 

 weights, and while both Avogadro and Amprere suggested a cor- 

 rect basis for the selection very shortly after Dalton's first publi- 

 cation, their suggestion did not meet with approval. So it hap- 

 pened that for the first half of the nineteenth century chemists 

 generally were confused about the atomic weights which they 

 should use, and many of them became skeptical about any possi- 

 bility of certain choice among the miltiples which might be 

 selected and were disposed to content themselves with a system 

 of equivalents. In the late fifties. Cannizaro, who died only two 

 years ago in Italy, rescued the law of Avogadro from oblivion and 

 contributed very much to the general acceptance of a rational 

 system of atomic weights. A decade later the discovery of the 

 periodic system by Mendeleef and Lothar Meyer confirmed in a 

 most brilliant manner the truth and. value of the principles on 

 which the new table of atomic weights was based. At the same 

 time the periodic system pointed, irresistibly, toward some com- 

 mon substratum or ultimate material from which the atoms have 

 been built up or into which they might be disintegrated, but this 

 remained for thirty years and longer only a tantalizing sugges- 

 tion. Meanwhile the insight which the development of organic 

 chemistry gave into the structure of molecules forced upon 

 organic chemists, at least, a growing conviction of the actual 

 existence of atoms and molecules. In spite of this, the close of 

 the nineteenth century saw the rise of an important school of 

 chemistry, under the lead of Ostwald, which apparently wished 

 to abandon the atomic theory altogether and would have been glad 

 to express all of the facts of chemistry in terms of energy and 

 in the language of mathematical equations. The discoveries of 

 the last decade seem to have convinced even the leader of this 

 school, and I think that to-day we may accept atoms and mole- 

 cules as actual existing entities with almost the same certainty 

 with which we accept the existence of the sun, moon and planets. 

 With the atomic theory goes, almost of necessity, the kinetic 

 theory of gases. Indeed, some of the most convincing demonstra- 

 tions of the truth of the atomic theory have come along the lines 

 of the kinetic theory. I might say, in passing, that the second law 

 of thermodynamics, that the entropy of a system always tends to 

 increase, presupposes, almost of necessity, an atomic constitution 

 of matter. 



