Supersaturation of Salt-Solutions. 455 
The question I have set myself is: What is a (so-called) 
supersaturated solution? And the answer I have obtained is 
a very simple, though at first sight strange one. Itis a non- 
saturated or just-saturated solution of the anhydrous salt. 
It is nearly three years since I made my first experiments 
on this subject, by determining the density of supersaturated 
solutions of sulphate and thiosulphate of sodium before and 
after crystallization; but at that time the results obtained had 
no meaning to me, and it is only recently that I have been 
able tointerpret them. The modus operandi.wasas follows:— 
A quantity of salt was placed in a weighed specific-gravity 
bottle, which was again weighed. A quantity of water was 
added, and the whole reweighed; then some parafiin-oil of 
known density was added, and the bottle was placed in hot 
water till complete solution was effected. After cooling, the 
bottle was filled up with paraffin, the stopper inserted, and 
then placed in a constant-temperature bath for some time, and 
finally dried and weighed. When the stopper was removed, 
a minute crystal of the salt under examination, when dropped 
through the paraffin into the solution, produced instant crys- 
stallization. After three or four hours in the constant-tem- 
perature bath crystallization was assumed to be complete, and, 
after refilling with paraffin, the bottle was again weighed. 
From the data thus obtained it was easy to calculate the 
density of the solution, and also that of the mixture of salt 
and saturated solution resulting from the crystallization. It 
