Solubility of Salts in Water. 11 



bility of mercuric chloride (HgCl 2 ) caused by the addition of 

 hydrochloric acid (with the common CI ion) to the solution. 

 For this and a number of allied cases the explanation has 

 been shown by Le Blanc and Noyes * to lie in the combina- 

 tion of HgCl 2 and HC1 to form double molecules, a different 

 substance with a different solubility. The determinations 

 were made by observation of the freezing-points of the 

 solutions, the lowering of the freezing-point being, as has 

 been shown by Raoult and others f, a function of the number 

 of molecules (or free ions) in the solution. 



Further work in this same direction % has applied, with 

 quantitative results, the Law of Mass Action to cases of the 

 mutual influence of the solubilities of pairs of salts which do 

 not possess a common ion. This work is of extraordinary 

 interest, since without the above indicated theoretical explana- 

 tion the observed phenomena would seem entirely without 

 meaning, the results could not in any way be foreseen. The 

 possibility of quantitative experiment is in this entire field 

 limited only by such concentrations which produce an osmotic 

 pressure in the solution not so great that the laws of gases no 

 longer hold. 



That these results already obtained agree over such a range 

 as that including inorganic salts and organic acids is strong 

 evidence for the soundness of the assumptions upon which the 

 work is based, more especially so since, so far as my informa- 

 tion goes, no other explanation whatever has been able to 

 account for the facts. 



It will be of interest to state that the above developed con- 

 siderations explain the action of a method used by Dr. Nicol 

 in purifying the chlorides and nitrates used by him, that of 

 precipitating the former by hydrochloric acid and the latter 

 by nitric acid. In this case the common ion CI or N0 3 added 

 decreases, in the way explained, the solubility of the salt, 

 which is consequently precipitated. In the same way, to 

 increase the insolubility of a precipitate a little sulphuric acid 

 is by analysts added to the wash-water in washing out lead 

 sulphate, or, in general, an addition of some of the precipitating 

 reagent to increase the insolubility of a precipitate. 



As bearing upon this question 1 take the opportunity to 

 communicate some results recently obtained by me § in a case 

 of mutual solubility where the formation of a double salt is 



* Le Blanc and Noyes, Zeitschr.fur phys. Chem. vi. p. 385. 

 t Raoult, Ann. chim. phys. [6] ii. p. 66; also Arrhenius, Zeitschr.fur 

 phys. Chem. i. p. 647. 



\ Noyes, Zeitschr.fur phys. Chem. vi. p. 262. 

 § Tyevor, Zeitschr. fur phys. Chem. vii. p. 468. 



