148 Royal Society : — 



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 

 49^ atmospheres the extra solubility was * 1 76 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-f/w, 

 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 be 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 1*045 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, 

 would be 157 atmospheres. Expressing this fact in other words, we 

 may say that a pressure of 157 atmospheres is the mechanical force 

 with which the salt tends to dissolve in a solution containing 1 per cent, 

 less than can dissolve at the same temperature without pressure, be- 

 cause the two forces exactly counterbalance one another. In a still more 

 dilute solution the force would of course be still greater, in accordance 

 with the fact of a greater pressure being necessary to prevent the salt 

 from being dissolved. Supposing then that we had a solution a trifle 

 more dilute than that just named, and in such indefinitely large 

 quantity that a cubic inch of the salt could dissolve in it and yet pro- 

 duce no sensible change in its strength, so that from first to last it might 

 be considered to dissolve under a pressure of 157 atmospheres, and also 

 supposing that it was rigidly enclosed on all sides but one, so that the 

 whole expansion must take place in one direction over an area of one 

 square inch, since on dissolving there is an increase in bulk from 100 

 to 115*78, the solution of this cubic inch would, as it were, raise 

 2355 lbs. through the space of -1578 inch. This is mechanically the 

 same as 371| lbs. raised 1 foot, or, the specific gravity of the salt 

 being 1*53, the same as 171 times the weight of the salt itself raised 



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



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



