[ 417 ] 



LVI. Note on Supersaturated Saline Solutions. 

 By Charles Tomlinsok, F.R.S.* 



THE current number of the Journal of the Chemical 

 Society has called my attention by its abstracts to two 

 most interesting papers on the above subject by Dr. Nicol, 

 contained in the Philosophical Magazine, 5th series, xix. 

 p. 453, and xx. p. 295. 



With Dr. NicoPs main conclusion, that a supersaturated 

 solution is a solution of the anhydrous salt, I cordially agree, 

 and expressed such an opinion in my paper read before the 

 Koyal Society in 1868. I also brought together in the 

 ' Chemical News ' of December 1869 various opinions of some 

 French chemists on the constitution of sodium sulphate in 

 solution. These are somewhat contradictory. The favourite 

 view of supersaturation in French, English, German, and 

 American textbooks long continued to be Lowel's, viz. that 

 a solution (say of sodium sulphate) saturated at a high tem- 

 perature, and left to cool out of contact with air, does not 

 actually become supersaturated, but undergoes a molecular 

 change, in which the 10-atom salt becomes the more soluble 

 7-atoni ; so that, instead of having a supersaturated solution 

 of the 10-atom salt, we have only a saturated solution of the 

 7-atom salt. 



Now it is remarkable that in Lowel's five memoirs, which 

 appeared the first in 1850 and the fifth in 1855, the above 

 theory is maintained in the case of sodium sulphate, and a 

 similar theory with respect to sodium carbonate and mag- 

 nesium sulphate. But in his sixth and last memoir, published 

 in 1857, he maintains that between the boiling-point of a 

 saturated solution of sodium sulphate (103 o, 17 C.) and the 

 point at which the 7-atom salt begins to be deposited (18°), 

 it is not this modified salt, but the anhydrous variety, which 

 is held in solution. He is led to this conclusion by con- 

 sidering : — 



1. That the 10-atom salt parts readily with its water of 

 crystallization by mere exposure to the air. 



2. That Faraday found, on evaporating a solution of the 

 10-atom salt below 100° C. (or, according to Mitscherlich, 

 below 40°), that the anhydrous salt was deposited. 



3. That in boiling for some time a saturated solution of the 



* Communicated by the Author. 

 Phil. Mag. S. 5. Vol. 21. No. 132. May 1886. 2 G 



