1863.] 545 



On comparing together the various salts, it will be seen that their 

 properties vary very considerably. Thus, under the same pressure, 

 the extra quantity of sulphate of copper dissolved in nearly ten 

 times that of ferridcyanide of potassium. The mechanical equivalents 

 also vary even more, being (for chloride of sodium) about 22| times 

 as great as for sulphate of copper. On the contrary, the mechanical 

 equivalents of ferridcyanide of potassium and sulphate of potash are 

 the same ; but, under equal pressures, the extra quantity of the 

 latter dissolved in nearly nine times as great, owing to the difference 

 in the amount of expansion in crystallizing. This latter is, however, 

 nearly the same for water and ferrocyanide of potassium, whilst, 

 under the same pressure, the extra quantity of that salt dissolved is 

 2J times that of ice, in consequence of the much greater mecha- 

 nical equivalent of the ice. It appears to me that we may pro- 

 visionally conclude that the increased solubility clue to pressure varies 

 directly with the change of volume, and inversely with the mecha- 

 nical equivalent of the force of crystalline polarity, so that, if S be 

 the total amount of salt which dissolves without pressure, c be some 

 function of the change in volume in dissolving, and m some function 

 of the mechanical equivalent of the force of crystalline polarity, the 

 solubility, at the same temperature, under a pressure of p atmo- 

 spheres would be S +-^-lf . If the salt be one that expands on dissolv- 

 ing, c of course is negative, and therefore under pressure the solubility 

 becomes S-^^; that is to say, it is diminished, as proved by ex- 

 periments with sal-ammoniac. If no change in volume took place, 

 we may, I think, also conclude that pressure would not in any way 

 increase or decrease the solubility of a salt. Moreover, since, when 

 a solution is just saturated, the force with which the salt tends to 

 crystallize is equal to that with which it tends to dissolve, their 

 mechanical equivalents must be equal and opposite. Hence we may 

 perhaps conclude that, other circumstances being the same, the 

 mechanical equivalent of a salt like chloride of sodium, which so 

 readily attracts moisture, would be greater than that of one like 

 sulphate of copper, which so readily loses even its water of crystal- 

 lization ; and thus also the relative influence of equal amounts of 



