RELATIONS TO PHYSICS AND CHEMISTRY 305 



(7) Pasteur (1853), Stereochemistry (1874). 



(8) Raoult, Arrhenius (1886-87). 



(9) Radioactivity (Becquerel, Curies). 



II. Ideas concerning Affinity 



(1) Berthollet, Guldberg, Waage (1867). 



(2) Berzelius, Helmholtz (1887). 



(3) Mitscherlich, Spring (1904). 



(4) Deville, Debray, Berthelot. 



(5) Thomson, Berthelot (1865). 



(6) Horstmann, Gibbs, Helmholtz. 



I. Physical Chemistry and our Ideas concerning Matter 



The Concepts of Atoms and Molecules. Regarded as a whole, we 

 may say that the initial application of physical knowledge for the 

 purpose of developing our ideas of matter consisted chiefly in the 

 employment of physical methods and instruments in the study of the 

 properties of matter. This stood foremost in physical chemistry in 

 the first period of its existence. 



Reviewing the history of chemistry, we must acknowledge that one 

 of the first fundamental steps was made by the study of the physical 

 property of weight, and the introduction of a physical instrument, 

 the balance, for this purpose. It was, in large part, on this basis that 

 Lavoisier was the great innovator of chemistry; and it was due solely 

 to the following of chemical change with the balance that chemistry 

 got its fundamental laws of constant weight and of constant and 

 multiple proportions. These were summarized by Dalton in the fruit- 

 ful though hypothetical conception of atoms, which, as is well 

 known to you all, asserts that every element exists in the form of 

 small unchangeable particles, identical for a given element, but 

 differing with the latter. 



As the study of weight led to the idea of atoms, so the study of 

 another physical property, that of volume and density, led to our 

 idea of molecules. These molecules, which might be described as con- 

 stellations of atoms, were a necessity with Dal ton's conception; but, 

 in a binary compound, for instance, they might consist of two atoms 

 or of twenty. Now, it hardly needs to be recalled that Gay-Lussac, 

 and especially Avogadro, in following the volume relations of gases 

 in chemical action, drew the conclusion that the molecules of gases 

 occupy equal volumes under identical conditions. Thenceforward 

 we had a reliable method for determining the relative weights of such 

 molecules. 



As the study of the physical properties weight and volume led to 

 the concepts of atoms and molecules, so sharply defined that the 



