Volumes of Salt -Solutions. 193 



Thorpe and Watts* have shown that the volume of water of 

 constitution in the solid state is less than the volume of the 

 water of crystallization ; so that were the water of crystalli- 

 zation attached to the salt in solution, we should expect to find 

 a very considerable increase of the molecular volume in solu- 

 tion ; but such is not the case. It is with very great hesita- 

 tion that I venture to suggest that this is a proof that water 

 of crystallization does not exist in solution ; there is really no 

 evidence that is conclusive as showing the opposite. At first 

 sight the experiments of Thomsenj and others, on the heat of 

 hydration of various salts, lend much support to the view that 

 water of crystallization does exist, as such, in a solution ; for 

 there is no doubt that while in very many cases a hydrated 

 salt dissolves in water with absorption of heat, the same salt 

 when dehydrated evolves heat on solution. But is it not pos- 

 sible that the act of solution of a dehydrated salt consists, first, 

 in the taking up of water to form a hydrate ; and that this 

 hydrate, in dissolving, parts with its water, which thus becomes 

 indistinguishable from the rest of the water ? When a solu- 

 tion, on the other hand, is concentrated, a point is reached at 

 which the salt is capable of carrying with it a certain number 

 of water-molecules, and then combination takes place. Some 

 support of this view is to be found in the fact that it is only 

 hydrated salts that are capable of forming well-defined super- 

 saturated solutions, and it is only on the supposition that the 

 molecules of water are not united to the salt that the existence 

 and density of such solutions can be explained. 



The experiments of Wiillner J on the tensions of salt solu- 

 tions led him to the conclusion that in some cases the water 

 of crystallization is attached to the salt in solution, in other 

 cases that it is not^ while KiidorfF§ and De Coppet|| have 

 come to conclusions differing from those of Wiillner, but not 

 agreeing among themselves. 



At this point I must leave this interesting question ; but I 

 am not without hopes of being able to solve it by experiments 

 on the vapour-pressures of water from salt-solutions, which I 

 have already commenced. 



* Chem. Soc. Journal, 1880. 



t Thermoch. Untersuch. vol. iii. 



| Pogg. Ann. vols, ciii., cv., and ex. 



§ Ibid. vols, cxiv., cxvi., and cxlv. 



|| Ann. Chim. Phys. 1871, 1872, 1875. 



Phil. Mag. S. 5. Vol. 18. No. 112. Sept. 1884. 



