326 Intelligence and Miscellaneous Articles. 



It contains therefore 51" 6 per cent, of water. Representing this 

 hydrate by Cr 2 O 3 , 9HO, theory would give 5T4 per cent, of water. 

 When a solution of chromite of potash is boiled for a few minutes, 

 it is decomposed ; a green precipitate of sesquioxide of chromium is 

 formed, which is entirely insoluble in the alkalies. This precipi- 

 tate was by repeatedly washing freed from potash, and yielded by 

 analysis — 



Hydrated oxide 0-729 



Water 0-360 



It contained therefore 49-3 per cent, of water. Representing this 

 hydrate by the formula Cr- O 3 , 8HO, theory would give 48*4 per cent, 

 of water. 



It appears, then, that the elimination of only one equivalent of 

 water is sufficient to completely modify the hydrate of sesquioxide of 

 chromium, and to render it insoluble in the alkalies. This hydrate 

 becomes anhydrous by heating to 266° F. 



This example will suffice to show the influence which water can 

 exert over the properties of a metallic oxide ; since, according to its 

 proportion, it determines its solubility or non-solubility in the alka- 

 lies ; it shows also the difficulties attendant upon the study of hy- 

 drates. M. Fremy states that he has often seen hydrates lose a cer- 

 tain quantity of their water at common temperatures during washing, 

 or even by the action of light alone, and thus acquire properties en- 

 tirely new. The instability of certain hydrates may be compared to 

 that of some metallic peroxides. 



The preceding hydrates are not the only ones which owe their 

 acid character to the presence of the water which they contain. The 

 hydrate of protoxide of tin, which dissolves, as well known, so readily 

 in potash and soda to form stannites, loses its solubility in the alka- 

 lies when it becomes anhydrous. 



The hydrates of the oxides of antimony, zinc and lead, which dis- 

 solve so readily even in very dilute alkaline solutions, can be dissolved 

 by them only when concentrated, if rendered anhydrous. 



It must then be admitted that hydrates often lose their acid cha- 

 racter on becoming anhydrous. As this modification of the properties 

 of a hydrate may be effected at common temperatures, and as it is 

 always accompanied with the disengagement of water, it cannot be 

 attributed to those isomeric changes which certain oxides undergo 

 when they are calcined, and which M. Chevreul has with reason 

 compared to the coagulation of albumen, and describes under the 

 general denomination of pMnonienes de cuisson. 



When a hydrated oxide dissolves in potash or soda, is it to be 

 understood that it combines with the base in the state of hydrate to 

 form a ternary group composed of water, of base and of acid ? In order 

 to resolve this question, it is requisite to obtain crystalline compounds 

 of the preceding hydrates with alkaline bases, and this M. Fremy 

 states he has not yet been able to accomplish. 



When a solution of an hydrated oxide in an alkali is suffered to 



