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PROFESSOR W. A. TILDEN AND MR. W. A. SHENSTONE ON THE 
however, strongly support a kinetic theory of solution based on the mechanical theory 
of heat. The solution of a solid in a liquid would accordingly be analogous to the 
sublimation of such a solid into a gas, and proceeds from the intermixture of molecules 
detached from the solid with those of the surrounding liquid. 
Such a process is promoted by rise of temperature, partly because the molecules of 
the still solid substance make longer excursions from their normal centre, partly 
because they are subjected to more violent, encounter with the moving molecules of 
liquid. Such a view does not necessarily involve the assumption of an “ attraction ” 
between the molecules of the solvent and those of the solvend (compare Dossios, 
Jahresb., 1867, 92 ; and Nicol, Phil. Mag., Feb., 1883). 
Indeed, it is difficult to disconnect “ attraction ” from the idea of combination 
resulting from such attraction. In some of the cases we are considering, as for 
instance the solution of anhydrous sulphate of sodium in water at 100°, nothing like 
combination between the water and salt seems to occur. 
We have satisfied ourselves by direct experiment that anhydrous sulphate of sodium 
at the temperature of 100° dissolves in water at the same temperature without any 
sign of previous combination, and the solution so prepared contains exactly the 
same amount of solid as the solution made by gradually heating up a solution 
prepared at a lower temperature. But when anhydrous sulphate of sodium is 
introduced into water below 34°, all the phenomena of combination are manifested, 
and the salt sets into a solid crystalline mass previous to dissolving. 
Whilst, therefore, we still think the act of hydration a factor in a great many 
cases of solution, it appears that it must be abandoned as a hypothesis of general 
applicability. 
We now proceed to describe the results we have obtained with other salts. 
Calcium sulphate.—Melting point a red heat. 
Sullivan (Hep. Brit. Assoc., 1859, 292) states that he has proved this salt to be 
insoluble in water at 150°, but we can find no detail of any experiments of his. 
Couste (Ann. des Mines [5], v., 140-144) describes experiments upon the solubility 
of calcium sulphate at high temperatures, but they appear to have been all made 
with sea-water, and there are many objections that might be raised to his mode of 
operating. 
In our experiments pure precipitated calcium sulphate, which had been most 
thoroughly washed, was used and distilled water. Glass is rapidly attacked by the 
solution, and the determinations were therefore made in the silvered metal tube. 
