Saturation of Salt- Solutions. 541 



I believe that further investigation will show the general 

 truth of the following conclusions: — 



That the more soluble a salt is in any liquid, the more nearly 

 will its mean molecular volume in the saturated solution 

 approach that in the solid state, and that in most cases the 

 product of F by the solubility by the molecular weight will 

 be found a constant, subject, however, to variations, due most 

 likelyjto the influence of the at present unknown factor attrac- 

 tion of salt for water. 



That the solubility of a salt is intimately connected with its 

 molecular volume is also shown by the following: — 



(1) Diminished molecular volume without change of com- 

 position is attended by diminished solubility: thus, in the case 

 of sodium sulphate, the salt crystallized at different tempera- 

 tures has a different molecular volume *. 



f =40° 



110° 



Melted 



M.V. =5335 



53-31 



53-96 



Solubilityf =48-8 



42-5 



J"230° 

 \46-4 



The solubility here is parts per 100 of water- 

 Under this head also come the instances given by me (loc. 

 cit.), and also the determinations of the rate at which CaC0 3 

 dissolves in dilute acids in the different forms of calcspar and 

 arragonite, where the former, with a lower specific gravity 

 (i. e. larger molecular volume), was found to dissolve fastest. 



(2) Diminished molecular volume with change of composi- 

 tion is attended by diminished solubility. 



Sodium sulphate has its maximum of solubility at 32°'8, 

 the temperature at which the decahydrated molecule is broken 

 up. So with sodium carbonate at 32°'5 (Mulder). 



Manganese sulphate has a maximum of solubility at 54° C, 

 and the solid salt decomposes into a lower hvdrate at 54° 

 (Tilden). 



Ferrous sulphate has its maximum at 63 0, 5, temperature of 

 decomposition (Tilden). Other cases are given by Mulder J. 

 But enough has been said to show that here the solubility is 

 connected with the hydration of the salt; but as increased or 

 diminished hydration means increased or diminished molecular 

 volume, the molecular volume is connected with the solubility. 

 I do not intend by the above to imply that in solution the 

 salt is combined with the water of crystallization. I formerly 

 held this, the general view; but experiments, nearly completed, 



* Phil. Mag. Feb. 1883, loc. cit. 



f Tilden and Shenstone, Proc. Roy. Soc. xxxv. p. 345. 

 X Mulder, Bijdragen tot de Geschiedenis van het scJieikundig gebonden 

 Water (Rotterdam, 1864). 



Phil. Mag. S. 5. No. 109. SuppL Vol. 17. 2 O 



