156 DISCUSSION OF EVIDENCE. 



and caesium. The salts of these elements generally crystallize without 

 water, and therefore have very little hydrating power in aqueous 

 solution. The approximate hydration 1 of salts of potassium has been 

 determined by the method usually employed and has been found to 

 be small. 



Some of the salts of lithium and potassium crystallize with 2 and 

 3 molecules of water, and these have been shown to have some hydrat- 

 ing power. 1 The atomic volumes of lithium and sodium are much 

 smaller than those of potassium, rubidium, and cesium. 



Turning from the maxima of the curve to the minima, at the mini- 

 mum of the third section of the curve are iron, cobalt, nickel, and 

 copper. Salts of these metals crystallize with large amounts of water, 

 and in aqueous solution they form complex hydrates. 



Aluminium falls at the second minimum of the atomic volume curve, 

 having a somewhat greater atomic volume than iron. The salts of 

 aluminium crystallize with large amounts of water, some of them with 

 6 and 8 molecules. In aqueous solution they form complex hydrates. 1 

 Barium has the largest atomic volume of members of its group ; its 

 salts crystallize without water or some with 2 molecules of water. 

 Many of the salts of calcium, strontium, and magnesium crystallize 

 with 6 molecules of water. Magnesium has the smallest volume of 

 any element of this group; it has been found to have the greatest 

 hydrating power of any member of the group. Strontium has a 

 slightly larger atomic volume than calcium and has a somewhat smaller 

 power to form hydrates. Taking all of the facts into account, it 

 would seem that, other things being equal, the smaller the cation the 

 greater its hydrating power. This raises the question, which ion is it 

 that forms the hydrate? Do both ions form hydrates. If so, which 

 has the greater hydrating power? 



The different salts of certain metals have approximately the same 

 hydrating power. The common constituent of these salts is of course 

 the cation, the anion varying from salt to salt. This would indicate 

 that it is primarily the cation which conditions the hydrating power 

 of a salt. Since the different salts of the same metal do not all have 

 the same hydrating power, it seems reasonable to assume that the 

 anion has some power to form hydrates in the presence of water. The 

 cation is, then, the chief hydrating agent, and its hydrating power 

 seems to be a function of its size or atomic volume the smaller the 

 ion the greater its power to hold water in combination with it in 

 aqueous solution. 



This raises the question, why is this the case? It has occurred to 

 me that the electrical density of the charge on the ion may have some- 

 thing to do with this relation. Other things being equal, the smaller 



'Carnegie Inst. Wash. Pub. No. 60. 



