Prof. J. P. Cooke on the Vermiculites. 245 



salts of the stronger acids the distinction between water of con- 

 stitution (basic water as it is sometimes called) and water of crys- 

 tallization is, as a rule, well marked. Water of crystallization 

 generally escapes at a comparatively low temperature, and the 

 loss is attended with a complete disintegration of the crystals, 

 which usually fall to powder. Water of constitution is held more 

 firmly, and the salt must be heated to a much higher tempera- 

 ture before it is expelled. Moreover, although the loss of basic 

 water may greatly alter the external aspect of the body, yet there 

 is not the same complete breaking up of the structure as before. 

 To illustrate these points, it is only necessary to refer to the 

 well-known reactions of the common rhombic sodic phosphate 

 when heated. 



It is generally believed that the water of crystallization exists 

 as such in the salt, the molecules of water forming a part of the 

 structure of the crystals; and the facts to be presented in this 

 paper tend to support this view. But our modern theories 

 assume that the so-called water of constitution, which may be 

 also driven off when the salt is sufficiently heated, is formed 

 under the influence of the heat from the atoms of hydroxyl, 

 H-0-, which are united to the acid radical of the compound. 

 But in every simple salt, whatever may be our theories in regard 

 to the mode of combination, the weight of the water of constitu- 

 tion, as well as that of the water of crystallization, must bear to 

 the weight of the salt a definite proportion, which can easily be 

 calculated from the symbol ; and the ratio between the amount 

 of hydrogen and the amounts of the other radicals must be that 

 of the atomic weights of these radicals or of their multiples. In 

 any given class of salts, moreover, in which the ratio between 

 the atomicity of the basic and acid radicals has a constant value, 

 all hydrogen which represents water of constitution will sup- 

 plement the other basic radicals, and added to them will com- 

 plete the relative amount of basic radical which the given class 

 of salts require ; while the hydrogen which represents water of 

 crystallization will be in excess of that amount. 



In mineral silicates these relations are complicated by the 

 phenomena of isomorphous replacements ; and although there 

 may be some question in regard to the replacing capacity of hy- 

 drogen, we cannot expect the same constancy, in the amount of 

 water of constitution which these minerals contain, as in the 

 case of simple salts. The amount of water of crystallization, 

 however, must be as invariable in one case as in the other, and 

 the hydrogen must bear the same relations to the atomic ratio 

 of the compound. Thus, if the mineral is an orthosilicate, in 

 which the atomic ratio between the sum of the basic radicals and 

 the silicon is 1 : 1, all hydrogen in excess of the amount which 



