The Heat of Dissolution of Gases in Liquids. 35 



and its significance is at once comprehended : a student's 

 idea of this quantity, or of the electric elasticity E, is often 

 very hazy, when formed from the definitions of the ordinary 

 method. Lastly, the method offers the student a new stand- 

 point, the view from which not only makes clear to him many 

 things that were confused before, but greatly assists in giving 

 him an insight into modern advances in electrical theory. 



III. The Heat of Dissolution of Gases in Liquids. 

 By Spencer Umfreville Pickering, F.R.S.* 



IF solutions consist of chemical compounds of the solvent 

 with the dissolved substance, the formation of these will 

 nearly always be accompanied by the liberation of heat : this 

 heat will, however, often be more or less masked by con- 

 comitant actions of a reverse nature, especially in cases where 

 the substance is a liquid or solid, the particles of which can- 

 not become resolved into the quasi gaseous molecules which 

 exist in solution without the absorption of a quantity of heat 

 corresponding with that of their heat of vaporization, or of 

 fusion plus vaporization. All such reverse actions, however, 

 would be absent if the dissolved substance were taken in the 

 gaseous state to start with, or, at most, there would only be a 

 small absorption of heat due to a slight separation of the par- 

 ticles of the solvent from one another ; this in very dilute 

 solutions would probably be negligible, and we should con- 

 sequently expect, if the hydrate theory of solution is correct, 

 that gases would dissolve with a considerable evolution of heat. 

 The following determinations will show that this undoubt- 

 edly is the case ; and whether this evolution of heat is due to 

 the formation of definite compounds or not, the fact that it 

 occurs must prove that solution is accompanied by a disap- 

 pearance of potential energy, that the dissolved substance 

 can no longer be represented as being in a condition of gaseous 

 freedom, and that the solvent cannot be' regarded as playing 

 the part of "so much space." The publication of these 

 results (most of which were obtained some time ago) comes, 

 I fear, rather late in the day ; for the " empty space " theory 

 would seem to be already abandoned by some of the sup- 

 porters of the physical theory of solution, and, indeed, has been 

 categorically repudiated by van der Waals (Zeitschr. f. phys. 

 Cliem. viii. p. 214, see ff. 188-222) in an important paper, in 

 which he arrives at the conclusion that there must be a specific 



* Communicated by the Author. 



D 2 



