260 THEORY OF SOLUTION AND CRYSTALLISATION. 



PCI3, and chlorine, Clg. At low temperatures near to its 

 condensing or liquefying point, a large proportion of penta- 

 chloride is present. But if heated this compound breaks up into 

 the two simpler molecules, PCI3, and CI2, the dissociation occurring 

 progressively with the rise of temperature until at 350°, or 

 thereabouts, it is complete. At that temperature the vapour 

 consists of a mixture in equal volumes of chlorine and the 

 phosphorus trichloride. But it has been shewn, on the one 

 hand, that if the liberated chlorine is gradually removed by 

 diffusion, the progress of the dissociation is much more rapid and 

 occurs at lower temperatures. Whilst on the other hand the 

 dissociation is retarded or almost arrested by causing the vapour of 

 the pentachloride to mix with a sufficient quantity of vapour either 

 of trichloride of phosphorus or of chlorine. 



Turning now again to solutions and, in order to avoid verbal 

 repetition, confining our remarks to solutions of salts in water, we 

 find a state of things of strictly analogous character though a 

 degree more complex. Suppose, for example, we dissolve some 

 ordinary sodium sulphate in a considerable quantity of water. The 

 salt first combines with water forming a hydrate, or rather, a 

 series of hydrates which are all richer in water than the ordinary 

 salt, containing ten molecules. These hydrates are liquid at 

 ordinary temperatures, and mix by the usual process of liquid 

 diffusion with the rest of the water. The chief of these hydrates 

 I believe to be none other than the cryohydrate. If the 

 temperature of the liquid is above the melting point of the 

 cryohydrate, this compound tends to split up into water and lower 

 hydrates. But if the amount of water present is comparatively 

 large, this tendency is counteracted, just as the disposition of 

 pentachloride of phosphorus to split into trichloride of phosphorus 

 and chlorine is reduced by the presence of excess of one or other 

 of those substances. Imagine now the solution of sulphate of 

 sodium heated, other conditions remaining unaltered. The amount 

 of original (cryo ?) hydrate diminishes as] the temperature rises. 



