1863.] 543 



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

 may say that a pressure of 1 57 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 Ibs. through the space of *1578 inch. This is mechanically the 

 same as 371J Ibs. 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 

 1 metre. Since it involves no arbitrary unit but the metre, I shall 

 adopt the last expression as the measure of the total amount of 

 mechanical work done by the solution of salts which expand in dis- 

 solving, and which may conversely be looked upon as the measure 

 of the mechanical force rendered latent and, as it were, expended 

 in the act of crystallization when crystals are deposited. The value 

 of this mechanical equivalent of course varies with the strength of the 

 solution, as already remarked. 



In the case of salts which occupy less space when dissolved than 

 when solid, pressure, like the increased temperature, causes them to be 

 more soluble ; mechanical force is lost when they dissolve, and is, as 

 it were, expended in giving rise to solution. When water thus con- 

 taining more of a salt than could otherwise be dissolved at the same 

 temperature is just saturated under any given pressure, the amount of 

 pressure represents the force of crystalline polarity tending to cause 

 the salt to be deposited in a crystalline form, but which is exactly 

 counterbalanced by that pressure. I will not give the details for each 

 salt, but subjoin a Table of the results at which I have arrived for such 

 as illustrate particular points of interest, the calculations being all made 



