580 FOURTH REPORT — 1834. 



zinc, is replaced by sulphate of potash, without any further 



change. The formula of the double sulphate is (K S) Z n S 6 + H. 

 In the double salt, the whole six atoms of water are retained 

 with somewhat greater force than in the simple sulphate ; but 

 even the double sulphate becomes anhydrous below 212° in 

 vacuo. 



The sulphates of the other metallic oxides mentioned are 

 quite analogous to sulphate of zinc in their habitudes with 

 water, although the particular temperature at which they part 

 with their water of crystallization is different in each. The 

 analogy holds also in the double sulphates of those oxides. 



Of hydrous sulphate of lime, or gypsum, the two atoms of 

 water which it contains appear to be essential, and are retained 

 at 212°. At a temperature not exceeding 270°, this salt becomes 

 anhydrous, but retains the power of recombining with two 

 atoms of water, or setting. The salt is then in a peculiar con- 

 dition. It is the debris of the hydrate, and not a neat chemical 

 compound. Heated above 300° the salt becomes properly 

 sulphate of lime, and has lost the disposition to combine with 

 water. 



The protochlorides, and corresponding cyanides of zinc, man- 

 ganese, iron, &c., are disposed to combine with two atoms of 

 water. Hence the cyanide of iron combines with two atoms 

 of cyanide of potassium, to form the double cyanide of iron and 

 potassium, commonly called the ferroprussiate of potash. 



Berzelius found the peroxide of tin formed by the action of 

 nitric acid on metallic tin, to differ in certain properties from 

 the same compound precipitated from a persalt of tin by an 

 alkali, and distinguished the first under the name of the nitric 

 acid peroxide of tin. Both peroxides combine with muriatic 

 acid, but the muriate of the nitric acid peroxide is peculiar in 

 being insoluble in water strongly acidulated with muriatic acid. 

 But the precipitated peroxide of tin assumes, I find, all the 

 properties of the other modification, when kept for some time 

 exposed to the heat of boiling water, or even when strongly 

 dried over sulphuric acid in vacuo, at the ordinary temperature 

 of the atmosphere. The two modifications are merely differ- 

 ent hydrates of the peroxide of tin, but it is difficult to ascer- 

 tain what proportion of water is essential to each. The hy- 

 drates, combine with acids, and form two sets of compounds ; 

 but absolute peroxide of tin itself (which is obtained by heat- 

 ing the hydrated peroxide to redness,) has no disposition to 

 combine with acids. The same is true of many other metallic 

 peroxides; they combine as hydrates only with acids. There 



