74 Royal Society : — 



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 crys- 

 tals 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 ; 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 in- 

 stantly crystallized, although another crystal was inactive to a solu- 

 tion 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 solution 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 eva- 

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

 of the solution, or a ring in the upper part of the vessel ; these crys- 

 tals are perfectly inactive, as has long been known -, and this has 

 been accounted for by regarding them as crystals of the modified 

 salt (Na 2 S0 4 , 7H 2 0), which is non-nuclear : but recently they have 

 been regarded as crystals of the normal salt (Na 2 S0 4 , 10H 2 O), and 

 their want of action upon the supersaturated solution has been ex- 

 plained 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 activity shown 

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

 and not as due to any property inherent in it. 



Lowel made analyses of this salt, formed by spontaneous evapo- 

 ration, and found it to consist of the modified salt containing 

 Na 2 S0 4 , 7H 2 0. 



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

 only that he gave it 8H 2 0, instead of 7H 2 0. There is no doubt 

 that Faraday obtained this salt and not the normal with 10H O, 



