June 8, 1900.] 



scienge: 



903 



published a series of quantitative studies of 

 osmotic pressure, using the copper ferrocyanid 

 cell, which was destined to be much used at a 

 later period. At not far from the same time, 

 ideas on the arrangement of atoms in space 

 began to germinate in the mind of van't HotF. 

 "From this he (van't HoiF) was led to study 

 reaction velocity, and from this the conditions 

 of equilibrium. But closely connected with the 

 problem of equilibrium was that of affinity. He 

 took up, as an example of affinity, the attraction 

 of salts for their water of crystallization, and 

 sought to measure this more directly than had 

 been done." It was at this point that his at- 

 tention was called to the work of Pfeffer, and 

 after studying concentrated solutions he turned 

 to dilute. In 1887 just ten years after Pfeffer's 

 work, van't HofT published a paper entitled 

 ' The Role of Osmotic Pressure in the Analogy 

 between Solutions and Gases,' and in this 

 shows the application of Boyle's law to dilute 

 solutions. The next step was to show that 

 Gay-Lussac's law also applied to dilute solu- 

 tions, and finally that these could only be true 

 on the assumption that the law of Avogadro was 

 equally true, that is "that solutions which at 

 the same temperature have the same osmotic 

 pressure, contain in a given volume the same 

 number of dissolved particles." 



It soon however became clear that "the os- 

 motic pressures of large classes of chemical sub- 

 stances do not conform to these laws. The 

 exceptions include all the acids, all the bases 

 and all the salts," that is electrolytes, and in all 

 these the osmotic pressure is greater than would 

 be expected. Thus far van't Hoff. The nest 

 step is taken by Arrhenius who reasoned that 

 as the anomalous vapor density of ammonium 

 chlorid, etc., was due to the dissociation of the 

 compound into simpler molecules, so in dilute 

 solution the salt is dissociated and this dissoci- 

 ation is into its ions, thus going back to Claus- 

 ius' theory of electrolysis. In this way came 

 into existence the theory of electrolytic dissoci- 

 ation as advanced by Arrhenius, which is briefly, 

 that all electrolytes when in aqueous solution 

 are dissociated to a greater or less extent into 

 their ions, which are the positive and negative 

 portion of the molecule laden with a charge of 

 electricity. 



Such is an outline of the development of the 

 theory as traced in this chapter, much of it in 

 the language of Pfeffer, van't HotF and Arrhen- 

 ius, and all of it lucid and logical. 



Chapter III. is devoted to a presentation of 

 the evidence for the theory. It is first shown 

 that the physical properties of completely dis- 

 sociated solutions should be additive, as is the 

 case with dilute solutions of strong salts. Then 

 the evidence from the heat of neutralization is 

 given. The next section is perhaps the most 

 important in the chapter, considering the rela- 

 tions between osmotic pressure, lowering of 

 freezing point, rise in boiling point and electri- 

 cal conductivity; itisjust here that to the older 

 chemists who have not been raised on this 

 theory, the evidence for it appears most con- 

 vincing, indeed it is the phenomena connected 

 with these points which it is most difficult to 

 account for on any other theory than that of 

 electrolytic dissociation. Experiments to show 

 the presence of free ions, effect of an excess of 

 one of the ions and the relation between disso- 

 ciation and chemical activity are the next sub- 

 jects considered and the chapter ends with the 

 effect of water on chemical activity. Numer- 

 ous examples are given, taken largely from the 

 work of H. Breretou Baker, showing the neces- 

 sity of the presence of at least a trace of water 

 in many chemical reactions. 



For instance, dry chlorin will not act on me- 

 tallic sodium ; dry hydrochloric acid will not 

 act on carbonates ; dry hydrochloric acid and 

 ammonia will not unite and dry sodium will 

 not decompose concentrated sulfuric acid. 

 Since the publication of the book, Baker has 

 succeeded in distilling phosphorous in an at- 

 mosphere of dry oxygen. The question is now 

 asked, ' ' Why is water essential " and an answer 

 is found " in that water has a very high dissoci- 

 ating power, breaking down the molecules into 

 ions which then react. These facts are just 

 what would be predicted if the theory of elec- 

 trolytic dissociation is true." 



Chapter IV. discusses first the application of 

 the theory of electrolytic dissociation to solu- 

 tions, and here are brought up a large number 

 of the problems most interesting the physical 

 chemists of to-day. This portion is particularly 

 vivid, so much of it is descriptive of the au- 



