Saturation of Salt- Solutions. 547 



soluble salt possesses a molecular volume the same as it 

 would if all the salt-molecules present were the same, and that 

 the less soluble salt possesses the molecular volume it has in a 

 saturated solution. There is here very complete agreement 

 between the two sets of figures. 



Enough has been said to show that in the majority of the 

 cases examined the molecular volume of the saturated solu- 

 tion is equal to the sum of the molecular volumes of the 

 salts present, calculated on the hypothesis that each salt has 

 the volume it would possess in its own saturated solution. 



The results of previous experimenters on this subject are 

 given in Table VII. I shall at present content myself with 

 pointing out the general conclusions to be derived from it, 

 reserving for future discussion the special points of diffi- 

 culty. I do so with the less hesitation, since the data here 

 given are liable to the same charges of inaccuracy that are 

 made with so much truth against ordinary solubility deter- 

 minations. 



Out of forty-two determinations (including duplicates) there 

 are thirty instances in which the more soluble salt has its solu- 

 bility increased by the presence of another salt to a greater 

 extent than that of the less soluble one. 



In no instance is the solubility of the more soluble salt less 

 than if it were alone in the solution, while in several instances 

 it is doubled. In twelve cases the less soluble salt suffers a 

 diminution of solubility; in all the others the reverse is the case. 



The general rule appears to be that the solubility of two 

 salts together is greater than that when separate. The pro^ 

 bable explanation of this appears to me to be as follows: — 



Each salt dissolves independently of the other. This is 

 shown by the molecular volume of the solution. Each salt, 

 however, increases in most cases the solubility of the other by 

 what maybe called mechanical means: thus, molecules of dif- 

 ferent salts do not, as a rule, attract one another; there is 

 therefore no tendency to form a system containing two salts. 

 At the same time, however, the formation of groups of similar 

 salt-molecules is prevented to a certain extent by the inter- 

 position or collision with a molecule of the other salt: thus 

 the number of similar salt-molecules must be increased before, 

 in unit time, the number of encounters of similar molecules 

 becomes as great as in a saturated solution of that salt alone. 



The increased solubility of the more soluble salt at the 

 expense, so to speak, of the less soluble salt is in all probabi- 

 lity due to the greater attraction for water possessed by that 

 particular salt. This is specially noticeable in the case of 

 CaCl 2 . 



