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SCIENCE. 



[N. S. Vol. XXII. No. 569. 



3. Dulong, Petit, Mitscherlich (1820). 



4. Faraday (18.32). 



.5. Bunsen, Kirclihoff (1861). 

 6. Periodio System (1869). 

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



8. Raoult, Arrhenius (1886-7). 



9. Radioactivity (Becquerel, Curies). 



ir. IDEAS CONCERNING AFFINITY. 



1. Berthollet, Guldberg, Waage (1867). 



2. Berzelius, Helmholtz (1887). 



3. Mitscherlicli, Spring (1904). 



4. Deville, Debray, Bertlielot. 



5. Thomsen, Berthelot (1865). 



6. Horstiuann, 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 knowl- 

 edge for the purpose of developing our 

 ideas of matter consisted chiefly in the em- 

 ployment of physical methods and instru- 

 ments in the study of the properties of 

 matter. This stood foremost in physical 

 chemistry in the first period of its ex- 

 istence. 



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 chem- 

 ical 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 

 fruitful 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 constellations 

 of atoms, were a necessity with Dalton '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 rela- 

 tions of gases in chemical action, drew the 

 conclusion that the molecules of gases oc- 

 cupy equal volumes under identical condi- 

 tions. 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 relative weights of these entities 

 form the fundamental constants of chem- 

 istry, so a further study of physical prop- 

 erties has led to broad generalizations con- 

 cerning the nature of atoms and molecules, 

 which we shall now outline. 



Properties of Atoms. — As to atoms, I 

 would call your attention to four peculiar- 

 ities which seem to me of fundamental im- 

 portance. First, Dulong and Petit found 

 that the physical property called heat 

 capacity is nearly the same for different 

 atoms, i. e., that the quantity of heat 

 requisite to produce a given rise of tem- 

 perature does not vary greatly for atomic 

 quantities, for 7 parts of lithium and for 

 240 parts of uranium. 



Second, Faraday, in studying the elec- 

 trical conductivity of electrolytes, e. g., of 

 aqueous solutions of salts, found that the 

 quantity of electricity which atoms can 

 transport varies as the whole numbers, — 

 from one in potassium to two in zinc. This 

 fundamental property, which gives the 

 sharpest expression to our notion of 

 valency, was brought by Helmholtz into a 

 very clear form by the assumption that 

 electricity as well as matter consists of 



