TEANSACTIONS OF SECTION B. 



655 



in polarised light. A comparison of the chloride, bromide, and iodide of didj'mium 

 has also been made, from which it appears that in the bromide the bands are 

 situated o\ further towards the red end of the spectrum than in the chloride, 

 whilst the displacement for the iodide is 14X towards the violet. In the solution 

 of the chloride (or nitrate) the bands have almost the same position as in the 

 crystals of the iodide, whilst the addition of nitric acid causes a displacement 

 of 12X towards the red. It is proposed to determine how far tliis displacement of 

 bands is due to the dispersion equivalent of the menstruum, and whether it gives 

 evidence of dissociation in the liquid. 



4. On. Solution. By William Durham, F.E.S.E. 



The object of this paper is to show that thermo-chemical results accumulated of 

 late years entirely support the theory of solution wliich the author brought forward 

 in a paper read "before the lioyal Societj^ of Edinburgh in January 1878, and 

 developed in subsequent papers.' 



That theory may be briefly described as follows. Solutionis due to the chemical 

 affinity of the elements of the substance dissolved for the elements of the solvent. 

 For instance, common salt, NaoCl„, dissolves in water because of the affinity of Na„ 

 for O and of CI., for IL. Further, chemical affinity is not in all cases exhausted 

 when definite compounds are formed, but sufficient remains to form Avhat may be 

 called ' solution compounds.' 



In support of this view it is pointed out that in all chlorides, bromides, iodides, 

 sulphates, and nitrates for which data are available the heat of solution varies 

 directly — 



(ij As the heat of combination of the positive element of the salt with in 

 water varies. 



(2) As the heat of combination of the negative element of the salt with H 

 varies. And inversely — 



(3) As the heat of combination of the positive and negative elements of the salt 

 varies. Examples are given, such as the following: — 



' Proceedings of the Royal Society of Edinburgh, Jan. 21, 1878; May 17, 1886; 

 Jan. 17, 1887; July 18, 1887. Nature, -vol. xxxiii. p. 615; vol. xxxiv. p. 263; vol. 

 sxxvi. p. 316. 



