On the Direct Correlation of Mechanical and Chemical Forces. 147 



class of facts to be taken into account, I shall base my conclusions on 

 my own experiments. The volume with which salts exist when in 

 solution, assuming that of the water to remain unchanged, varies 

 greatly in the case of different salts, and also according to the 

 amount in solution and the temperature. Thus, taking sal-am- 

 moniac as an example, when there are 3 per cent, in solution the 

 volume is as if it expanded 3 - 40 per cent, on dissolving; whereas 

 when 25"55 per cent, are in solution, the expansion is 11*36 per 

 cent. ; and when nearly concentrated at about 13° C, an additional 

 quantity expands on dissolving 15' 78 per cent. In by far the greater 

 number of cases, however, there is a contraction on dissolving, 

 and the amount gradually diminishes for each additional quantity 

 entering into solution, so that the mean result is very different from 

 what occurs when the solution is dilute or is nearly saturated. It is 

 this contraction or expansion when a small additional quantity is dis- 

 solved in a nearly concentrated solution that must be taken into 

 account in the following calculations. 



In determining the influence of pressure on the solution of salts, I 

 found it requisite to adopt somewhat different methods according to 

 the peculiarities of the salts. In some cases I sealed up in a saturated 

 solution portions of the salt in clean, solid crystals, and determined 

 the effect due to pressure from their loss in weight ; whereas in other 

 cases I sealed up solutions containing more salt than could be dis- 

 solved at the temperature at which the experiments were made, and 

 determined the effect of pressure from the difference in the weight 

 of the crystals deposited ; being of course careful to make allowance 

 for any difference in the amount of solution in the tube with pressure 

 and in that without, and to avoid any error that might be produced 

 by a different temperature. In all cases I have had a tube with 

 pressure and another without, treated from first to last in precisely 

 the same manner, and kept at exactly the same temperature, so that 

 pressure was the only difference ; and usually the effect was so well 

 marked that there was no doubt about the result. In the case of 

 chloride of sodium, solution goes on so slowly, and the mechanical 

 equivalent of the force of crystallization is so great, that if pressure 

 had been applied for only a few hours one might have concluded, 

 with Bunsen, that pressure has no influence on solubility ; but, by 

 maintaining it for a week or more, there was no difficulty whatever 

 in perceiving that a solution which was quite saturated without pres- 

 sure, dissolved more under a pressure of about 100 atmospheres. 



The solubility of a salt in water appears to me to result from a kind 

 of affinity which decreases in force as the amount of salt in solution 

 increases. This affinity is opposed by the crystalline polarity of the 

 salt ; and when the two forces are equal, the solution is exactly satu- 

 rated. As is well known, a change in temperature alters this equi- 

 librium ; and, according to my experiments, mechanical pressure rela- 



1859. Tissier, L'Institut, 1859, p. 158; 1860, p. 281. Kremers, Pogg. Ann. 

 vol. lxxxv. pp. 37 and 246 ; vol. xciv. p. S7 ; vol. xcv. p. 110 ; vol. xcvi. p. 39 ; 

 vol. cv. p. 360 ; vol. cviii. p. 1 1 5. Billet, These de Chemie. 



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