RECENT ADVANCES IN SCIENCE 631 



as it indicates that hydration of the solute is a general 

 phenomenon, a conclusion which has been reached on other 

 grounds. 



INORGANIC CHEMISTRY. By C. Scott Garrett, D.Sc. 



Water of Crystallisation. — As we have already pointed out in 

 these quarterly reports, the whole subject comprising what 

 is somewhat loosely termed " water of crystallisation " is one 

 which so far has not received the attention which it merits. 

 Chemists are prone to be satisfied with merely stating the 

 empyrical number of molecules of water or other solvent occur- 

 ring in a compound without trying to differentiate between 

 them or allotting to them specific structural positions. The 

 solution of these and other problems in this field, besides being 

 interesting, will furnish considerable aid in advancing our 

 knowledge of structure and reactivity in inorganic chemistry. 



Some work in this connection has recently been published 

 by Guareschi (Atti R. Accad. Sci. Torino, 191 5, 50), who has 

 made measurements of the amount of water driven off from 

 different types of hydrates over various temperature ranges 

 and under various conditions. 



In the case of dihydrated calcium sulphate, for instance, 

 both the natural variety and that obtained by precipitation 

 dehydrate at 93-94 , or in a current of dry air at 81-82 , but 

 whereas the former dehydrates readily, the latter loses water 

 very slowly. And, as is well known, a moderately stable 

 semi-hydrate is obtained which, in the absence of further 

 data, probably possesses a composition represented by the 

 formula Ca 2 S 2 8 .H 2 0. Sodium nitroprusside crystallises with 

 empyrically two molecules of water, and when dehydrated it 

 loses water readily at 98-99 up to an amount corresponding to 

 one and a half molecules of water. The final half molecule of 

 water is only separated slowly and with difficulty, and for this 

 reason the hydrate is probably correctly represented by the 

 formula [Na 2 Fe(NO)(CN) 5 ] 2 .4H 2 0. Moreover, the reactivity 

 of the hydrate molecules in this salt would seem to vary with 

 the conditions under which the hydrate has been formed, 

 for when the salt has been dehydrated and allowed to rehydrate 

 itself, it then can be dehydrated again much more easily. 



Many so-called dihydrates lose their water of hydration 



