Solubility of Salts in Water. 373 



general. He has not, however, succeeded in obtaining a 

 single one of these double salts in the solid form and in 

 definite crystals ; and it will be found, on comparing his results 

 with those of Riidorff, that the difference of a few degrees in 

 the temperature of experiment is sufficient to effect a radical 

 change in the proportions of the two salts, an effect which is 

 extremely improbable if such double salts really existed in 

 solution. Mulder's double salts, in all probability, owe their 

 supposed existence to the arithmetical possibility of fitting any 

 experimental ratio very closely by ratios such as 1:2, 2:3, 

 3:4, 4:5, &c. 



Riidorff*, the latest experimenter on the mutual solubility 

 of salts when dissolved to saturation, has performed an im- 

 mense number of very careful and exhaustive experiments not 

 only with salts between which chemical action is impossible, 

 but also with those where double decomposition may take 

 place. We are here concerned only with the former. He 

 found that these divide themselves into two groups. With 

 some salt mixtures, as for instance nitrate and chloride of 

 ammonium, we obtain always one and the same saturated 

 solution, if only an excess of both salts be employed, no matter 

 in what relative proportions the excess of both salts is brought 

 in contact with the water. With other salts, however, such 

 as ammonium and potassium nitrates, the composition of the 

 resulting solution is dependent on the proportions in which 

 the two salts are presented to the water, which is insufficient 

 to completely dissolve either of them. An excess of the one 

 drives out a definite amount of the other salt, so that it is not 

 possible with these salts to prepare a saturated solution on 

 which one or other of the salts is without action. 



In his later paper on the same subject, he extends his 

 experiments to many other pairs of salts, with the general 

 result that, so far as we are here concerned, it is only those 

 salts which are not isomorphous nor form double salts, nor form 

 mixed crystals, that are capable of forming definite saturated 

 solutions — solutions, the composition of which is unaffected 

 by the addition of either of the salts in the solid state. A 

 fuller account of Riidorff 's later work has already been given 

 by me in a paper on the " Saturation of Salt Solutions." 



The work of the other experimenters on this subject does 

 not call for any special comment, as the greater part of it refers 

 to individual cases, and is not sufficiently extended either in 

 its aim or execution to allow of any generalization being made 

 from it. 



Though so much has been done by the four investigators 

 * Wiedemann's Annalen, xxv. p. 626. 



