RELATIONS TO OTHER SCIENCES 81 



of ferric chloride by Roozeboom, and of the crystallization out of 

 sea-water of the contained salts by van't Hoff and Meyerhoffer 

 indicates the great value of the phase rule in bringing scientific order 

 out of the complicated relations of the various components and 

 phases involved. 



Speaking of this department of physical chemistry, van 't Hoff re- 

 marked, " Since the study of chemical equilibrium has been related 

 to thermodynamics, and so has steadily gained a broader and safer 

 foundation, it has come into the foreground of the chemical system, 

 and seems more and more to belong there." And Ostwald says in 

 answer to the question, " What are the most important achievements 

 of the chemistry of our day? I do not hesitate to answer: chemical 

 dynamics, or the theory of the progress of chemical reaction, and 

 the theory of chemical equilibrium." 



These statements, coming from two masters in the field, are most 

 significant of the importance of the introduction of these ideas into 

 chemistry. 



The conceptions and methods of physical chemistry have also been 

 most strongly felt in the field of electrochemical theory. To the 

 question what is the nature of electrolysis, Faraday and Hittorf 

 and Clausius had each contributed important elements of the final 

 answer, then came Arrhenius with the theory of electrolytic dissocia- 

 tion, which has proved so fruitful of consequences, not only in the 

 domain of chemistry, but also in biology and in physics. 



One of the most interesting scientific questions connected with 

 electrochemistry is the relation between electromotive force and 

 electrolytic separation, and the development of the theory of the 

 voltaic cell. The question of the seat of electromotive force in the 

 cell was for many years the very storm-centre of physical discussion; 

 but from the standpoint of electrolytic dissociation Nernst has sup- 

 plemented the work of Helmholtz and Gibbs, and out of all has come 

 a theory which, while not perfect, seems to be in its main features on 

 the solid foundation of the conservation of energy and the laws of 

 thermodynamics. 



Another important service for which the world of science is indebted 

 to physics is the determination of the absolute zero of temperature 

 in terms of degrees of the ordinary centigrade scale. About a century 

 ago, Dalton, in his new chemical philosophy, adopts 3000 C. as the 

 probable zero of temperature. While Lavoisier and Laplace make 

 various estimates of the zero ranging from 1500 to 3000 degrees 

 below the freezing-point of water. But when the doctrine of energy 

 became firmly established together with the kinetic theory of gases, it 

 was natural that the condition of a gas in which the particles had no 

 energy of motion, and hence no pressure, should have been taken as 

 indicating the absolute zero. But it was Clausius and Lord Kelvin who 



