October 23, 1903.] 



SCIENCE. 



523 



our iiiaiu question. The real point to note 

 is that the physical properties of the ele- 

 ments are all interdependent, and that the 

 fundamental constants are the atomic 

 masses. 



Do I seem to exaggerate? Then look 

 for a moment at the present condition of 

 physical chemistry, and see how moderate 

 my statements really are. We have not 

 only the laws already mentioned, of Avo- 

 gadro, of Dulong and Petit, of Faraday 

 and of Mendeleeff, but also a multitude of 

 relations connecting the physical constants 

 of bodies with their chemical character. 

 Even tlie wave-lengths of the spectral lines 

 are related to the atomic weights of the 

 several elements, as has been shown by the 

 researches of Runge and his colleagues, of 

 Runimel,* and of Marshall Watts. f If we 

 try to study the specific gravity of solids 

 or liquids, the only clues to regularity are 

 furnished by the atomic ratios. Atomic 

 and molecular volumes give us the only 

 approximations to anything like order. 

 Similarly, we speak of atomic and molec- 

 ular refraction, of molecular rotation for 

 polarized light, of molecular conductivity 

 and the like. In Trouton's law, the latent 

 heat of vaporization of any liquid becomes 

 a function of the molecular weight. And, 

 finally, all thermochemical measurements 

 are meaningless until they have been 

 stated in terms of gram molecular 

 weights ; then system begins to appear. 

 Chaos rules until the atomic or molecular 

 weight is taken into account ; with that con- 

 sidered, the reign of order begins. 



Even to the study of solutions the same 

 conditions apply. Substances in solution ex- 

 ert pressure, and in this respect they closely 

 resemble gases. Van't Hoff has shown 

 that equal volumes of solutions, having 

 under like conditions equal osmotic pres- 



* Proc. Roy. Soc. Victoria, Vol. 10, part I., p. 15. 

 t Phil. Mag. (6), .5, 203. 



sures, contain equal numbers of molecules, 

 and thus Avogadro's gas law is curiously 

 paralleled. The two laws are even equiva- 

 lent in their anomalies. The abnormal 

 density of a gas is explained by its dissocia- 

 tion, and the variations from van't Hofif'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, how- 

 ever, are atoms or groups of atoms; and 

 just as Avogadro's law is applied to the 

 determination of molecular weights among 

 gases, so van't Hoff's rules enable us to 

 measiu'e the molecular weights of sub- 

 stances in solution. The atom, the mole- 

 cule, and the molecular weight enter into all 

 of these new generalizations. In short, 

 if we take the atomic theory out of chem- 

 istry, we shall have little left but a dust- 

 heap of unrelated facts. 



I have now indicated, briefly and in out- 

 line 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 purpose, 

 and they make, at least for chemi.sts, an 

 exceedingly strong case. The convergence 

 of the testimony is remarkable, and when 

 we add to the chemical evidence that which 

 is offered by physics, the theory becomes 

 overwhelmingly strong. This side of tlie 

 question I can not attempt to discuss, but 

 I may in passing just refer to Professor 

 Riicker's presidential address before the 

 British Association in 1901, which covers 

 the ground admirably. The atomic theory 

 has had no better vindication. 



And yet, 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 science. Some of the objectors 



