542 [April 30, 



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

 sure, dissolved more under a pressure of about 1 00 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- 

 tively increases one or other of these opposing forces, according to the 

 mechanical relations of the salt in dissolving. At all events in the 

 case of chloride of sodium the extra quantity dissolved under pres- 

 sure varies directly with it for such pressures as glass tubes will resist, 

 in the same manner as, according to Thomson's experiments, the 

 fusing-point of ice is reduced. Thus I found that for a pressure of 

 49i atmospheres the extra solubility was '176 per cent., and for 121 

 atmospheres "431, which are almost exactly in the same ratio. Hence, 

 if S be the amount soluble without pressure, under a pressure of p 

 atmospheres the solubility at the same temperature would be S +ps> 

 where the values of S and s are independent, and vary for different 

 temperatures and different salts. Future experiments may perhaps 

 show that this conclusion should be modified ; but yet it will be well 

 to adopt it provisionally, in order to compare together the mechanical 

 relations of different salts which otherwise would not be so intelligible. 



According to Michel andKrafft* and to Schifff, sal-ammoniac is 

 the only salt known for certain to occupy more space in solution than 

 when crystallized. Hence under pressure mechanical force must be 

 overcome in dissolving, and experiment shows that, on this account, 

 the relative force of crystalline polarity is increased and the solubi- 

 lity decreased. This is the reverse of what results from an elevation 

 of the temperature, so that the effect cannot Ije due to heat generated 

 by the pressure, but must be the direct consequent of pressure. Calcu- 

 lating from an experiment where the pressure was 164 atmospheres, 

 which gave a decreased solubility of T045 per cent, of the whole salt 

 in solution, a pressure of 100 atmospheres would cause '637 per cent, 

 less to be dissolved than is soluble at 20 C. without pressure, and the 

 pressure requisite to reduce the solubility to the extent of 1 per cent. 



* Ann. de Chim. 3 ser. vol. xli.p. 471. 



f Ann. der Chemie, vol. cix. p. 325 ; vol. cxiii. p. 329. 



