the Nature of Liquids. 131 



to the second view, additional work is done in dissociating the 

 complex molecules into their simpler constituents, and in 

 imparting increased velocity and internal motion to those 

 constituent molecules (see " Evaporation and Dissociation," 

 part i., Trans. Koy. Soc. 1886, Part L). 



When a substance, such as chloral hydrate or ammonium 

 chloride, passes from the solid or liquid into the gaseous state, 

 the physical change is obviously accompanied by a chemical 

 one, for dissociation into simpler molecules occurs. There is 

 an obvious analogy between evaporation and such cases of 

 dissociation ; and we have recently undertaken experimental 

 work to test whether this analogy is a real one. 



In part i. of this series of papers the phenomena attending 

 the volatilization of such solids as dissociate wholly or partially 

 on their passage from the solid to the gaseous state have been 

 studied. There are two ways of measuring the vapour- 

 pressure of a stable substance, which have been termed by 

 Regnault the statical and the dynamical respectively. The 

 first consists in measuring the pressure exercised by the 

 vapour of the substance kept at a uniform temperature ; and 

 the second in measuring the highest temperature attainable 

 by the substance at given pressures, when evaporation freely 

 takes place. It has been shown by Regnault, and by nume- 

 rous other observers, that these methods give identical results 

 with liquids, and by ourselves with solids (Trans. Roy. Soc. 

 Part I. 1884, p. 37). But in the case of the majority of the 

 dissociable bodies examined, the results of the two methods 

 were not identical; indeed, in many cases in which dissociation 

 is complete, or nearly so, the temperature of volatilization is 

 independent of pressure. With nitrogen peroxide, acetic 

 acid, and ammonium chloride, however, the two methods 

 gave identical results. This method, therefore, cannot be re- 

 garded as a means of deciding the question of the analogy 

 between evaporation and dissociation, unless, indeed, two 

 kinds of chemical combination be conceived, one of which 

 may be termed " molecular combination" as distinguished 

 from " atomic combination." 



In parts ii. and iii. the thermal behaviour of stable liquids 

 has been investigated, as exemplified by alcohol and ether. 

 For a complete account of these researches reference must be 

 made to the original papers (Trans. Roy. Soc. 1886, Part I.*). 

 We are here concerned chiefly with the densities of the 

 saturated vapours, and with the heats of vaporization. We 

 found, with alcohol, that the density of the saturated vapour 

 was normal at temperatures below 40° or 50°, and remained 

 * The constants for ether will be published shortly. 



