374 Dr. W. W. J. Nicol on the Mutual 



mentioned above in their endeavour to ascertain the laws of 

 mutual solubility of salts, it does not seem to have occurred to 

 any one that only one, and that the least promising side of the 

 question, had been attacked. With the exception of a few 

 incidental experiments of RiidorfF, the attention of chemists 

 has always been directed to the saturated solutions of both 

 salts ; no one has realized the importance of experiments on 

 the solubility of a salt in a non-saturated solution of another ; 

 and yet a moment's thought will suffice to convince one that 

 it is in this way, and in this way only, that the problem of 

 mutual solubility can be solved. The influence of a salt on 

 the solubility of another cannot possibly be studied, if only 

 one determination be made of the condition of equilibrium in 

 the system of two salts and water. No matter how frequently 

 or how accurately this one determination is made, it will 

 throw no light on the question under examination. Several 

 years ago, soon after the publication of my previous paper on 

 the subject, I was struck with the futility of the methods 

 hitherto adopted, and commenced a series of experiments on 

 an entirely fresh field of investigation ; the progress of this 

 work has been reported on from time to time to the British 

 Association for the Advancement of Science by the Committee 

 " On the Nature of Solution," of which I have the honour to 

 be Secretary. 



The work had for its object the determination of the 

 solubility of one member of a pair of salts in solutions of 

 various strengths of the other member, and vice versa. In 

 this way it was hoped that the series of experiments on solu- 

 tions of gradually increasing strengths would throw light on 

 the action of the one salt on the solubility of the other, and 

 thus lead to a full knowledge of the condition of equilibrium in 

 the saturated solution of both salts. How far this hope has 

 been fulfilled will be seen below. 



The course of experiment was as follows : — 



A salt was taken, and solutions containing 1, 2, &c. gramme- 

 molecules of this salt in 100 gramme-molecules of water were 

 prepared. These solutions were then saturated at a definite 

 temperature with the other salt added in excess, and the total 

 salt in solution determined ; precisely similar determinations 

 were made with definite molecular solutions of the other salt. 

 The solubility of each salt separately in water at that tempe- 

 rature was then determined, and, finally, the solubility of both 

 salts when simultaneously added in excess to water was also 

 ascertained. 



The first difficulty met with was that of ensuring saturation; 

 for the conditions of experiment necessitated the greatest care 



