IX PROCEEDINGS. 



eoveries must revolutionize chemical theories and conceptions. It 

 is my purpose this evening to attempt to show very briefly what 

 the position of the great central theory of modern chemistry is in 

 the light of the most recent investigation. 



It is just a year more than a century ago that Dalton published 

 to the world the first full account of his atomic theory, in order 

 to explain the laws of chemical combination which he had himself 

 helped to formulate. The idea that matter has a grained structure, 

 or is composed of minute particles more or less distant from one 

 another, was advanced by one of the early Greek philosophers more 

 than twenty-four centuries ago, and thereafter similar specula- 

 tions had been from time to time entertained by various poets, 

 philosophers and scientists. But to Dalton is due the credit of first 

 applying these ideas to the explanation of chemical laws, and 

 thereby converting an idle metaphysical speculation into a fruitful 

 scientific theory. The fundamental fact which the atomic theory 

 has to explain is that the combination of elements with one another 

 not only takes place in certain invariable proportions but also that 

 these proportions can all be expressed as integral multiples of cer- 

 tain numbers, one for each element. Thus, the only proportions 

 in which oxygen is known to combine with other elements can be 

 expressed by 16 multiplied by 1, or 2, or 3, or some other whole 

 number. The proportions in which carbon is found iri any of its 

 hundred thousand compounds can always be expressed by 12 or 

 some integral multiple of 12. These are the facts. Now it is 

 plain that these facts receive a simple explanation if we suppose 

 that each element is composed of minute particles, all of constant 

 weight for the same element, and that chemical combination takes 

 place between these particles. This was Dalton' s atomic hypo- 

 thesis. According to it, then, elementary matter resembles those 

 articles of commerce that we can only buy in cakes or parcels of a 

 definite weight, like soap. Different elements correspond to differ- 

 ent brands of soap, each brand being made up into cakes of a 

 uniform weight, but of different weight from the cakes of any 

 other brand. The association of one or more cakes of one brand 

 with one or more cakes of any other constitutes chemical com- 

 bination. Now let us suppose our cakes so minute that they are 



