444 REPOET— 1886. 



On the Phenomena and Theories of Solution. 

 By Professor W. A. Tilden, F.R.S. 



[A communication ordered by the General Committee to be printed in extenso 



among the Reports.] 



In what follows I propose to review the principal phenomena ob- 

 served in the act of solution of solids, and especially of metallic salts 

 and other comparatively simple compounds in liquids, with the object of 

 arriving, if possible, at some conclusion as to the physical explanation of 

 the facts. For want of time and space an exhaustive statement of all 

 that is known cannot be attempted. The most important parts of 

 the subject omitted are the following : — Electrolysis, formulae relating to 

 expansion and density of solutions, their absorption-spectra, and other 

 optical properties, and magnetic rotatory power. 



One question which must arise in the course of the discussion is 

 whether the phenomena are to be accounted as chemical or physical, and 

 this necessarily involves another question, namely, what is chemical 

 combination, and by what criterion can it be distinguished from adhesion 

 or cohesion, or other manifestation of molecular or molar attraction ? 

 Postponing this enquiry for the present, we niay assume that chemical 

 combination has generally been supposed to be distinguished by definite 

 proportions in weight and volume of the acting masses, by definite 

 thermal changes, and by marked differences between the properties of the 

 compound and those of its components. 



The various theories which have been proposed to explain the nature 

 of solution are roughly divisible into two classes, namely, those which 

 represent solution as a kind of chemical combination, and those which 

 explain the phenomena by reference to the mechanical intermixture of 

 molecules, or by the influence of the rival attractions of cohesion in the 

 solid and liquid, and of adhesion of the solid to the liquid. 



The older writers seem universally to have regarded the act of solu- 

 tion as a manifestation of chemical attraction. Thus Henry (' Elements 

 of Chemistry,' 11th ed., 1829, vol. i. Chap. II.) refers to the solution 

 of common salt in water as 'one of the simplest cases that can be adduced 

 of the efficiency of chemical affinity, for solution is always the result of 

 an affinity between the fluid and the solid which is acted upon, often 

 feeble it is true, yet sufficient in force to overcome the cohesion of the 

 solid. This affinity continues to act until at length a certain point is 

 attained where the affinity of the solid and fluid for each other is balanced 

 by the cohesion of the solid, and the solution cannot be carried further. 

 This point is called saturation, and the fluid obtained is termed a saturated 

 solution.' 



Tui-ner, in his ' Elements of Chemistry ' (1842), also attributes solution 

 to ' the exercise of chemical attraction.' 



Gay-Lussac (' Ann. Chim. Phys.' xi. 297) states that ' the solubility 

 of a body in water depends on two causes, affinity and heat ; or more 

 exactly the affinity of salt for water varies with the temperature.' 



BerthoUet, in his ' Statique Chimique,' gives an elaborate statement 

 of his view, which is too long to quote in full, but the substance of 

 which is to set forth the influence of affinity in bringing about solution 



