146 DISCUSSION OF EVIDENCE. 



Taking all of these factors into account, it still seems highty probable 

 that, by the method outlined above, we can arrive at a reasonably 

 close approximation to the amount of water combined with a molecule, 

 or the resulting ions, of a dissolved substance, under given conditions 

 of concentration. Whatever objection may be offered to this method 

 of calculating the approximate composition of the hydrates existing 

 in aqueous solution, it should be stated that it is the only general 

 method thus far worked out for throwing any light whatever on 

 this important problem. Jones and Getman applied this method of 

 calculating the approximate composition of hydrates to about 100 

 compounds salts, acids, and organic substances and to about 1,500 

 solutions of these substances. Their results have been recorded in 

 Publication No. 60 of the Carnegie Institution of Washington. 



Salts of lithium form more complex hydrates than those of sodium 

 and potassium. This would be expected, since lithium salts crystallize 

 with water, while the salts of the other alkalies in general crystallize 

 without water. 



Salts of potassium and ammonium generally crystallize without 

 water, and these compounds, as would be expected, combine with rela- 

 tively little water in aqueous solution. 



Many salts of sodium crystallize without water, and these hydrate 

 very slightly. Other sodium salts, such as the bromide and iodide, crys- 

 tallize with water and show considerable hydrating power in solution. 



Salts of calcium crystallize with water and all have, as would be 

 expected, large hydrating power. The halogen salts crystallize with 

 6, the nitrate with 4 molecules of water. The nitrate was found to 

 have less hydrating power than the chloride or bromide. 



The salts of strontium resemble those of calcium, both in the amounts 

 of water with which they crystallize and with which they combine in 

 aqueous solution. Salts of barium crystallize with less water and 

 show less hydration than those of calcium and strontium. 



The salts of magnesium have just about the hydration that would 

 be expected from their water of crystallization. The same may be 

 said of the salt of zinc that was studied. 



Cadmium is of special interest. Its halogen compounds crystallize 

 with little or no water, and although cadmium belongs in the same 

 group with metals of large hydrating power, its halogens combine 

 with only a small amount of water. The nitrate of cadmium crystal- 

 lizes with 4 molecules of water and, as could be predicted, shows con- 

 siderable hydrating power. 



The chloride and nitrate of magnesium show the hydration that 

 would be expected from their water of crystallization. The same may 

 be said of the salts of nickel, cobalt, and copper. 



The chlorides and nitrates of aluminium, iron, and chromium crys- 

 tallize with large amounts of water and show great hydrating power. 



