306 PHYSICAL CHEMISTRY 



relative weights of these entities form the fundamental constants of 

 chemistry, so a further study of physical properties has led to broad 

 generalizations concerning the nature of atoms and molecules, which 

 we shall now outline. 



Properties of Atoms. As to atoms, I would call your attention to 

 four peculiarities which seem to me of fundamental importance. First, 

 Dulong and Petit found that the physical property called heat ca- 

 pacity is nearly the same for different atoms, i. e., that the quantity 

 of heat requisite to produce a given rise of temperature does not vary 

 greatly for atomic quantities, for 7 parts of lithium and for 240 parts 

 of uranium. 



Second, Faraday, in studying the electrical conductivity of electro- 

 lytes, 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 atoms, either negative or posi- 

 tive, and that material atoms are able to combine with them, potas- 

 sium with one of the positive kind, zinc with two, chlorine with a 

 negative one, - - and so transport them in electrolysis. 



The third great step was made by the study of light, a physical 

 property again. Bunsen and Kirchhoff found that, heated in the gas- 

 eous state, every atom emits a definite set of light-waves, producing a 

 characteristic line-spectrum which is yet the sharpest test of the kind 

 of atoms one is dealing with, and which so became the most fruitful 

 guide in the detection of new kinds. 



The last generalization that I have to mention, and which we owe 

 to Newlands, Mendeleeff, and Lothar Meyer, includes physical pro- 

 perties in general, and asserts that they vary with increasing atomic 

 weight in a periodic way. This shows itself most sharply in the atomic 

 volume, which passes through maximum values in lithium (7), sodium 

 (23), potassium (39), rubidium (85), and caesium (133). A correspond- 

 ing periodicity is observed in other properties, as, for example, that 

 of combining with electrical atoms, or valency, which in the said ele- 

 ments passes through unity. Analogous behavior is exhibited by the 

 melting-points and boiling-points, which for these metals are excep- 

 tionally low. 



If my programme did not to a certain extent exclude quite recent 

 investigations, confining me to a view of past history, I should like to 

 consider one more physical property, that of radioactivity, which also 

 seems to be a property of atoms. I can only insist on the fact that it 

 was physical properties again, the making the air conductive for elec- 

 tricity, and the spectrum, which revealed radium. 



Properties of Molecules. Turning to molecules, I have three pre- 



