SOLUBILITY OF SALTS IN WATER AT HIGH TEMPERATURES. 
29 
discovery, because it seems impossible, by appeal to the commonly received theories of 
solution, to find a satisfactory explanation of all the facts of the case. If we admit that 
sodium sulphate, placed in contact with water at temperatures below 34°, dissolves 
in virtue of its power of entering into union with water to form liquid hydrates, 
the diminished solubility above that temperature must be due to dissociation of 
these hydrates, and production of the anhydrous salt, which is apparently much less 
soluble. What then is the cause of the much greater solubility of the ordinary 
crystals in which the salt is already united with a large quantity of water, and how 
can we explain the fact that the anhydrous salt increases in solubility in accordance 
with the common rule when the temperature is raised ? The explanation appears to 
be found in the difference of fusibility of the two compounds, NagSO^lOHoO melting 
at 34°, and Na 3 S0 4 , which melts at 860°. 
There is nothing new in the idea that readiness to melt by heat is associated with 
disposition to dissolve by contact with liquid solvents, for even so far back as 1819 we 
find Gay Lussac quoting with approval a still older explanation given by Lavoisier 
(‘Traite Elem. de Chimie.,’ ii., 39)* of the action of heat in causing increase of solubility. 
B ut we are not aware that it has been definitely brought to the test of experiment 
before. 
Supposing a substance heated with a solvent to the melting point of the former, 
three cases might present themselves :— 
(a.) The liquids might be miscible in all proportions. 
This is true of sulphate of sodium at 34°. The melted salt, Na 3 SO 4 10HoO, may be 
mixed with an indefinitely small quantity of water, or in other words is infinitely 
soluble. 
We have also ascertained that it is nearly true of benzoic acid, which melts at 120°. 
This compound is stated to be soluble in about 600 parts of water at 0°, in 200 parts 
at 18°, in 25 parts at 100°. By sealing it up with water in a glass tube and heating 
to a few degrees beyond the melting point, intermixture occurs in all proportions, and 
the liquid so obtained on cooling to 120°, or about 1° lower, becomes turbid from 
deposition of oily drops, which, however, immediately crystallise. 
(b.) The solvent might become saturated and the excess of undissolved substance 
remain over in a liquid state. 
(c.) Or both might become saturated, the one with the other, forming two distinct 
liquids. 
This occurs in the case of phenol (carbolic acid) and certain of the fatty series of 
acids, besides other well-known substances. 
Supposing either b or c to occur, the two liquids become miscible when the 
temperature is raised, as may easily be shown in either of the cases referred to. 
But the connexion between fusibility and solubility, though proved, does not wholly 
explain the nature of the initial stage in the process of solution of a solid. It does, 
* In the reprint of Lavoisier’s works, vol. i., p. 305. 
