304 HISTORY OF CHEMISTRY. [LECTURE XV. 



hydration of the salts, respectively, were indicated by the abrupt 

 variations of the pressure. 25 



Pfaundler 2(5 endeavoured to explain the, at first, surprising 

 fact of a partial decomposition which gradually increased with 

 rise of temperature, involving, as it did, the different behaviour 

 of similar molecules under the same conditions. Naumann 27 

 further developed these views, and they were more definitely 

 formulated by Horstmann, 28 who made use of Maxwell's pro- 

 bability theory 29 of the distribution of the velocities. 30 A close 

 agreement between this theory and the observations was noted 

 in various cases. 



Horstmann was the first who tried to establish a general 

 theory of dissociation, 31 starting from the principles of the 

 mechanical theory of heat especially from the so-called second 

 law. This was found to be in complete accord with the results 

 of experiment in one case. 32 The researches of Gibbs 33 and of 

 Helmholtz, 34 which were based upon similar principles, were 

 more comprehensive and highly productive. 



These investigations, which belong, in part, to the domain 

 of physics, have become of great importance with respect to 

 the question we have here to consider. Shortly after the first 

 researches of Deville, the opinion was stated by three different 

 chemists Cannizzaro, 35 Kopp, 36 and Kekule 37 that the so- 

 called abnormal vapour densities were to be explained as due 

 to the substances concerned breaking up into two or more con- 

 stituents. The latter were supposed to re-combine on cooling, 

 so that no decomposition was perceptible upon distillation. 



The difficulties which thus stood in the way of a direct 



25 Debray, Comptes Rendus. 66, 194 ; G. Wiedemann, Pogg. Ann. 

 Jubelband 1874, 474. w Pogg. Ann. 131, 60. <27 Annalen. Supple- 

 mentband 5, 341. w Berichte. I, 210. <29 Phil. Mag. [4] 19, 22 ; 

 35> l %5- 30 Compare also Boltzmann, Wiedem. Ann. 22, 31. 31 Ann- 

 alen. Supplementband 8, 112 ; 170, 192. ** Ibid. 187, 48. 33 Silli- 

 man's Journal. 16, 441 ; 18, 277. a4 Berlin. Akad. Ber. 1882, 22, 825 ; 

 1883, 647. 35 Nuovo Cimento. 6, 428 ; 7, 375 ; 8, 71. Compare also 

 Repert. de Chimie pure. I, 201. 36 Annalen. 105, 390. :!7 Ibid. 106, 

 142. 



