ON THE PHENOMENA AND THEORIES OF SOLUTION. 



463 



weak ones, and that in the cases examined the solubility is greater 

 the more nearly the molecular volume in a saturated solution ap- 

 proaches the molecular volume of the salt in the solid state. But it has 

 not, so far as I know, been shown that the molecular volume in the solid 

 state determines the degree of solubility of the salt. Thus the molecular 

 volume of KCl solid is 37-4, and the molecular volume of NaCl solid is 

 27 1. But sodium chloride is more soluble at temperatures below about 

 25° than potassium chloride, and silver chloride, which is insoluble, has 

 a molecular volume, 2.5'8, scarcely less than that of sodium chloride. 

 Similarly, the molecular volume of KNO3 (solid) is 48-7, and that of 

 NaNOj, 37'9, but the latter is more soluble than the former. 

 Molecular volume is dependent — 



(1) On the atomic weights of the elements present. 



(2) On the constitution of the substance, that is, on the manner in 

 which the constituent atoms are united together. 



(3) On the density of the substance. 



It has been already shown that in many cases increase of molecular 

 weight corresponds to diminished solubility. 

 In the equation 



Mol. weight 

 Density 



the value of molecular volume is greater in proportion as molecular 

 weight is greater, but also in proportion as density is less It is the 

 latter which seems to coiTespond to increased solubility. 



(4) Molecular volume is also probably connected with fusibility 

 and perhaps with hardness as distinguished from density.' But 

 neither molecular volume nor fusibility is sufficient to determine the 

 degree of solubility of a solid. There must be another element in the 

 question which may be, and probably is, attraction or affinity — what- 

 ever that may mean — between the substance and the solvent. 



This is shown by such a case as the nitrates of potassium, sodium, 

 and silver : — 



Mol. vol. = 



' The mineralogists' ordinary scale of luirdru'ss, with the exception of talc, re- 

 presents a rough scale of solubility: — 1, talc; 2, rock salt ; 3, calc spar; 4, fluor ; 

 5, apatite ; 6, felspar ; 7, quartz ; 8, topaz ; 9, sapphire ; 1 0, diamond. 



