448 REPORT — 1 886. 



said to be saturated. . . . Although in the majority of instances the 

 solubility of a substance is increased by heat, it is not uniformly so. 

 (Exceptions quoted : lime, sulphate and sucrate of lime, sulphate and 

 seleniate of soda, &c.). These anomalous results may be partly explained 

 by the consideration that heat diminishes the force of adhesion as well 

 as that of cohesion ; generally speaking cohesion is the more rapidly 

 diminished of the two, although not uniformly so, and in the cases of 

 which we are now speaking it would appear that the adhesive force 

 decreases in a greater ratio than the cohesion of the saline particles.' 



The same idea forms the basis of the theory which has been supported 

 so actively by the writings and experimental researches of Dr. W. W. J. 

 Nicol (' Phil. Mag.' Feb. 1883, &c.). 



Dr. Nicol's view is stated in the following passage : — ' The solution 

 of a salt in water is a consequence of the attraction of the molecules of 

 water for a molecule of salt exceeding the attraction of the molecules 

 of salt for one another. It follows, then, that as the number of dissolved 

 salt molecules increases, the attraction of the dissimilar molecules is more 

 and more balanced by the attraction of the similar molecules ; when these 

 two forces are in equilibrium saturation takes place ' (Feb. 1883). 



L. Dossios has made use of the kinetic theory of Clausius relating to 

 the constitution of bodies and the process of evaporation as the basis 

 of a theory of solution. If we assume the kinetic energy of two 

 neighbouring molecules to be less than their attraction, such molecules 

 remain at a determinate distance from each other. This is the solid state. 

 The gaseous condition is assumed when the kinetic energy of a molecule 

 overcomes the combined attractions of the other molecules present. In 

 a liquid the energy of two neighbouring molecules is sufficient to enable 

 them to overcome each other's attraction, but is not equal to the united 

 attractions of the surrounding molecules. Relations of the same kind 

 may be supposed to exist in an aggregation of dissimilar molecules such 

 as compose a solution. 



Two liquids are miscible in all proportions when the attraction of 

 dissimilar molecules is capable of overcoming the attraction of similar 

 molecules, &c. 



The solution of solid bodies in liquids may be reduced to the same 

 principles. As, however, the attraction of the molecules of solids to one 

 another is large, and the kinetic energy destroyed, a solution of a solid 

 cannot be formed in all proportions, but a point of saturation is attained, 

 whilst the solubility of solid bodies increases generally with the tempera- 

 ture, since the action of heat is always opposed to molecular attraction 

 (' Jahresbericht,' 1867, p. 92). 



A physical theory, which differs from those referred to above in not 

 requiring the assumption of an attraction of either chemical or mechanical 

 nature between the molecules of the solvent and those of the solvend, was 

 briefly enunciated in a paper communicated to the Royal Society by 

 TUden and Shenstone in 1883. In discussing the connection between 

 fusibility and solubility of salts, the authors point out that the facts tend 

 to ' support a kinetic theory of solution based on the mechanical theory 

 of heat. The solution of a solid in a liquid would accordingly be 

 analogous to the sublimation of such a solid into a gas, and proceeds 

 from the intermixture of molecules detached from the solid with those of 

 the surrounding liquid. Such a process is promoted by rise of tempera- 

 ture, partly because the molecules of the still solid substance make longer 



