1872.] Mr. A. Liversidge on Supersaturated Saline Solutions. 505 



Exp. Supersaturated solutions of common potash alum were treated in 

 the same way and with the like results ; alum, perhaps, affords a prettier 

 example even than sodic sulphate, since the crystals formed in the tube 

 are of an opaque white, and can therefore be more readily observed during 

 their growth. 



Exp. Supersaturated solutions of magnesic sulphate were also operated 

 upon and with the same success j but the experiment is not so striking, 

 owing to the much longer time required by magnesic sulphate to crystallize. 



Although pure clean crystals of the normal sodic sulphate are active to 

 a supersaturated solution of sodic sulphate, yet. as might be expected, they 

 are not active to a similar solution of alum or magnesic sulphate, and vice 

 versa. 



For example, let us take a supersaturated solution of alum, and one of 

 sodic sulphate, and also crystals of both their salts, which crystals have 

 just formed and are taken from their still warm mother-liquors. 



Exp. A crystal of alum from its mother-liquor was added to a super- 

 saturated solution of alum. Crystallization immediately took place. 



Exp. A like crystal of alum was then added to a supersaturated solution 

 of sodic sulphate. No effect. 



Exp. A crystal of the normal salt was taken from its mother-liquor and 

 added to a solution of sodic sulphate. The solution instantly crystallized, 

 although another crystal was inactive to a solution of alum. 



Exp. A crystal of magnesic sulphate was added to solutions of alum and 

 of sodic sulphate respectively. No effect on either, but active in a solu- 

 tion of magnesic sulphate. 



Concerning the composition of the crystals of sodic sulphate formed by 

 spontaneous evaporation of a supersaturated solution of the same. 



When a supersaturated solution of sodic sulphate is allowed to evaporate 

 spontaneously, a crust or ring of crystals forms on the surface of the solu- 

 tion, or a ring in the upper part of the vessel ; these crystals are perfectly 

 inactive, as has long been known ; and this has been accounted for bv re- 

 garding them as crystals of the modified salt (Xa 3 S0 4 , 7H 2 0), which is 

 non-nuclear : but recently they have been regarded as crystals of the normal 

 salt (Na a S0 4 , 10H a O), and their want of action upon the supersaturated 

 solution has been explained by saying that, unlike crystals which have 

 been exposed to the air, they are chemically clean, and therefore free from 

 any film of greasy or other matter; for this writer views the acthity 

 shown by the normal salt as being entirely due to impurity of this kiud, 

 and not as due to any property inherent in it. 



Lowel made analyses of this salt, formed by spontaneous evaporation, 

 and found it to consist of the modified salt containing Na , S0 4 , 7H O. 



Faraday also examined it and came to much the same conclusion, only 

 that he gave it 8H a O, instead of 7H a O. There is no doubt that Faraday 



