THE ATOMIC THEORY. 255 



and of Marshall Watts. ^ If we try to study the specitic gravity of 

 solids or liquids, the only clows to regularity are furnished by the 

 atomic ratios. Atomic and molecular yolumes give us the onl}^ 

 approximations to anything like order. Similarly, we speak of atomic 

 and molecular refraction, of molecular rotation for polarized light, of 

 molecular conductivity, and the like. In Trouton's law the latent heat 

 of vaporization of an}' liquid becomes a function of the molecular 

 weight. And, iinally, all therniochemical measurements are meaning- 

 less until they have been stated in terms of gram molecular weights; 

 then system begins to appear. Chaos rules until the atomic or molec- 

 ular weight is taken into account; with that considered, the reign of 

 order begins. 



Even to the study of solutions the same conditions apply. Sub- 

 stances in solution exert pressure, and in this respect they closely 

 resemble gases. Van't Hot! has siiown that equal yolumes of solu- 

 tions, having under like conditions equal osmotic pressures, contain 

 equal numl)ers of molecules, and thus Avogadro's gas law is curiously 

 paralleled. The two laws are even equivalent in their anomalies. The 

 abnormal density of a gas is explained by its dissociation, and the 

 variations from van't Hotl's law are explicable in the same way. The 

 theory of ionic or electrolytic dissociation, proposed, by Arrhenius, 

 shows that certain substances, when dissolved, are split up into their 

 ions, and through this conception the analogy between gases and 

 solutions is made absolutely complete. The ions, however, are atoms 

 or groups of atoms, and just as Avogadro's law is applied to the deter- 

 mination of molecular weights among gases, so van't Hoft's rules enable 

 us to measure the molecular weights of substances in solution. The 

 atom, the molecule, and the molecular weight enter into all of these 

 new generalizations. In short, if we take the atomic theory out of 

 chemistry we shall have little left ])ut a dust heap of unrelated facts. 



1 have now indicated briefly, and in outline only, the influence of the 

 atomic theory upon the development of chemical thought. Details 

 have been purposely omitted; the salient facts are enough for my pur- 

 pose, and they make, at least for chemists, an exceedingly strong case. 

 The convergence of the testimony is remarkable, and when we add to 

 the chemical evidence that which is oflfered l)y physics, the theory 

 becomes overwhelmingly strong. This side of the question I can not 

 attempt to discuss, but I may in passing just refer to Professor 

 Riicker's presidential address ])efore the British Association in 1901, 

 which covers the gi-ound admirably. The atomic theory has had no 

 better vindication. 



And 3'et from time to time we are told that the theory has outlived 

 its usefulness, and that it is now a hindrance rather than a help to 



«Phil. ]\Ia^'. (6), 5, 203. 



