286 REPORT—1885. 
termed supersaturation is not really so at all. Thus a distinctly super- 
saturated solution of sodium sulphate readily dissolves a quantity of 
the dehydrated salt when brought in contact with it without access 
of air. This shows that the solution is not even saturated, much less 
supersaturated; still this may be explained by the supposition that the 
constitution of a supersaturated solution is not the same as an ordinary 
one, inasmuch as heat is necessary for its preparation; the effect of 
heat being to decompose the decahydrate, no union of water and salt 
taking place in cooling. In the second paper it is shown that this 
is entirely a mistake. Supersaturated solutions are readily prepared 
in the cold by simply enclosing the dehydrated salt in a bulb, placing 
this in a bottle with the proper quantity of water, and, after closing, 
heating the bottle to 100° for a few minutes. When the whole is cold, 
the bottle is shaken, the bulb broken, and the salt readily dissolves. If 
excess of salt be used, the solution has the same percentage composition 
as one prepared by heating the decahydrate, and allowing it to cool with 
the excess of salt to the same temperature, air being excluded. It is 
further found that when the dehydrated salt is brought in contact with 
the water, as above described, no caking together is observable, the 
powdery condition being retained till solution is complete. Thus there 
is no hydration previous to solution, as is indeed shown by the possibility 
of preparing supersaturated solutions in this way, for were the smallest 
trace of the decahydrate produced such a solution, could not be formed. 
During the act of solution, however, considerable heat is evolved, which, 
as shown above, cannot be due to hydration, but may possibly result 
from the enormous contraction, about 40 per cent., undergone by the 
Salt. 
Finally, density determinations of solutions of Na,SO, and Na,S,0,, 
of various strengths, show that in passing the ordinary saturation point 
there is nothing to indicate any change in the constitution of the solution. 
The molecular volume steadily increases from the most dilute solution 
up to the most concentrated supersaturated solution examined, exactly 
as it does with an ordinary solution which is not capable of super- 
saturation. 
From these and other experiments it follows that a so-called super- 
saturated solution is simply a saturated or non-saturated solution of 
the anhydrous salt; that any solution of a hydrated salt contains no 
hydrate of that salt, but that it is at the moment of crystallization that 
combination of the water and salt takes place. 
IV. Vapour Pressures of Salt Solutions. 1. Boiling Points of 
Saturated Solutions.! 
The method of experiment was to measure the pressure under which 
a saturated solution of the salt boiled at a definite temperature. 
The experiments included determinations at 65°, 75°, 85°, and 95° for 
NaNO, KNO;, Na.CO3, K,CO3, MnSO,, FeSO,, and the results are 
expressed in terms of degrees of rise of boiling point. This is found 
to be a quantity increasing with the temperature when the solubility 
increases ; on the other hand, it decreases when the solubility diminishes 
with rise of temperature. 
It is preferable, however, to express the effect of salt on the 
1 Published Phil. Mag., October 1885. 
