318 Dr Ogden on Saline Crystallisation. 



virtue of the known power possessed by saturated solutions to 

 dissolve a little of another different salt. I had no reason to 

 doubt the purity of the salt which I employed, and I tried many 

 different specimens, and always obtained the same result. To 

 put it still further to the test, I prepared the solution as above 

 described, and when cool, poured the supersaturated liquid in- 

 to another vessel^ and crystallized it. I then returned the mo- 

 ther water into the flask containing the undissolved salt ; but it 

 had no solvent effect on it. So that the very same salt which 

 would have dissolved in a cold supersaturated solution, was in- 

 soluble in a solution simply saturated, which is the condition of 

 mother water after the deposition of crystals. 



Doubtless this power of solution possessed by the supersatu- 

 rated liquid, has its limits ; as the first stage, or simple satura- 

 tion, is the limit of the solvent power of cold water ; but what 

 is the hmit of supersaturation, yet remains to be ascertained. 

 And the inquiry is beset with some difficulties; for, independ- 

 ently of the great habiiity of strong solutions entirely to crystal- 

 lize, the process is sometimes interrupted by the deposition of 

 the brilliant quadrangular crystals, containing eight proportionals 

 of water of crystaUization. Sulphate of soda may indeed be 

 considered capable of three stages of saturation ; the first is the 

 limit of the solvent power of cold water ; the second is the li- 

 quid which has deposited quadrangular prisms ; and the third 

 contains a still greater quantity of salt. The next experiment 

 illustrates these three stages. 



13. A supersaturated solution of sulphate of soda, with a por- 

 tion of undissolved salt remaining at the bottom of the vessel, 

 was allowed to stand at rest for four days. It was then briskly 

 asitated, and most of the salt was dissolved. All this time it 

 had existed in the third stage of saturation, although not at the 

 limits of that stage. The next day there had been a consider- 

 able deposition of brilliant and transparent crystals. The re- 

 maining liquid was now in the second stage of saturation. The 

 whole was again repeatedly shaken in the course of three hours, 

 without effecting any change. Two hours afterwards, the liquid ^ 

 suddenly became nearly solid, without any apparent cause ; the 

 small quantity of fluid now remaining was in the first stage of 

 saturation. 



