ON SUPERSATURATED SOLUTIONS. I39 



and the solutions absorb water from the air greedily. — If a drop of 

 a strong solution is put on blotting paper, and then this warmed 

 over a flame so as to make the salt anhydrous and is then hung up, 

 a dark spot shows itself in about ten minutes, which dries up 

 instantly it is touched with a crystal. Drops of strong solutions 

 on glass rapidly increase in size, but I do not think that they ever 

 absorb water beyond the limit of supersaturation. When a crystal 

 is introduced, it takes some time — a couple of hours perhaps, or 

 more — before the crystal is seen to have increased much in size. 

 They might, perhaps, do so in very wet weather. This strong 

 afl^nity for water seems to be the reason why absorbents have so 

 little effect on the solutions, only acting on the very strongest ones. 



The Carbonate stands next in order : the drysalt effloresces, but 

 not at all readily, unless the air is very dry -, the solutions on glass 

 slowly evaporate, giving the modified salts : boiling the solution 

 does not separate any anhydrous salt, only a monohydrate. Hence 

 absorbents act much more readily on these solutions than on the 

 acetate, giving the normal salt in strong solutions and the modified 

 salts in weaker ones, while still weaker ones are not affected at all. 



The sulphate both in the dry state and in solution is very easily 

 rendered anhydrous. It effloresces quickly, and boiling gives- 

 abundance of anhydrous salt. Hence absorbents act very readily 

 on it, and it always crystallises on blotting paper however weak the 

 solution is. 



Aerial Nuclei. — The fact that absorbent substances cause 

 crystallisation having been thus established, it is evident that a 

 considerable amount of light is thrown 'on the sensitiveness of 

 these solutions, and especially of sodium sulphate. Aerial nuclei 

 may now in many cases be supposed to be light absorbent particles. 

 These have no effect on the acetate at all, and I never cover 

 solutions of this salt however strong. They would act only on 

 strong solutions of the carbonate, and as a matter of fact, I never 

 cover solutions unless they contain three or more parts of salt to 

 one of water. 



