130 On Supersaturated Saline Solutions. [Jime 15, 



theory is true, the order o£ sensitiveness of the solutions should be 

 the order of comparative rarity of the salts; and this remains to be 

 proved. 



As to the cause of supersaturation, a good many facts seem to show 

 that it is the anhydrous salt which enters into solution. The lower 

 hydrates seem to be first formed, as in the case of sodium sulphate and 

 the alums. In the case of the hyperacid sodium sulphate with two parts 

 acid to one of salt, repeatedly boiled, it seems to be the anhydrous salt 

 which is first deposited. When the aqueous solutions of sodium sul- 

 phate and the alums are made to crystallize, the modified salts become 

 opaque white, while the hyperacid salt remains unchanged, and can be 

 obtained unchanged by heating the opaque variety from the top so as to 

 dissolve this, but not the anhydrous. 



Against the theory that it is the anhydrous sodium sulphate in 

 solution at low temperatures must be set the following fact. 



Lowel, in his Tables of the solubility of the thi-ee forms of sodium 

 sulphate, which are found in all our text-books, gives 412 parts of salt 

 to 100 of water as the maximum solubility of the 10-atom salt ; and this 

 is the highest number for any of the three kinds. K"ow I have dis- 

 solved 600 parts of 10-atom salt in 100 of water at 37° C. without 

 throwing doAvn a trace of anhydrous. I then warmed it : at 45° a 

 doubtful trace of anhydrous : at 51° very few ; at 60° still very few ; at 

 67^ about as much as would he on a little-finger nail ; at 75° eight or 

 ten times as much, the liquid nearly opaque ; at 80° a large quantity ; 

 boiled, the salt thickly covered the bottom of a large flask. 



Xow here the solution at 60° practically retained the whole of the 6 oz. 

 of salt to 1 of water, while according to Lowel it should have retained 

 only 2^ oz. 



Then between 70° and 80° a sudden change takes place, and a large 

 quantity is thro^m down. This agrees so far with Lowel's Table, as, 

 accordiug to him, at 84° the whole of the excess was practically thrown 

 down. This looks very much like dissociation taking place at that tem- 

 perature ; and that would involve the supposition that it was the 10-atom 

 salt in solution before. The difference in our results springs from the 

 different modes of working. Lowel always mamtained a large excess of 

 anhydrous present, whilst I added the salt in small portions, carefully 

 avoiding throwing do^^^l any anhydrous. This is pretty easily done by 

 keeping up a very rapid motion so as to prevent the liquid from getting 

 heated too much at auy point. It seems to me that in any case, as the 

 six ounces fairly dissolved, the solubility of the 10-atom salt should be 

 given in those proportions. Further experiments would, I have no 

 doubt, give still higher figures. 



In conclusion, I would remark that if the crystal theory of these 

 solutions be accepted we have a test of great delicacy in these drops for 

 the presence of the salts. Interesting experiments might be made as 



