PERSALTS 



dens vegetable blues. This salt, abstracting the 

 water which could not be determined, is com- 

 posed of 



1 atom sulphuric acid 7*5 

 1 atom peroxide of iron 5 



12'5 



Pertetra- g. Pertetnisulphate of iron. The insoluble 



sulphate, . . . 



portion remaining after the persesquisulphate of 

 iron has been extracted, is a red coloured pow- 

 der destitute of taste, and insoluble in water. 

 It is anhydrous, and composed of 



1 atom sulphuric acid 5 



4 atoms peroxide of iron 20 



25 



It is, therefore, a pertetrasulphate of iron. 

 Pcrsesqui. 3, Persesquinitrate of iron. This salt forms 



nitrate, 



large and pretty regular crystals. It was first 

 noticed by Vauquelin, above 20 years ago, who 

 formed it by leaving nitric acid for a long period 

 in contact with black oxide of iron. The crys- 

 tals form spontaneously in the liquid. The 

 crystals, when first formed, are transparent and 

 colourless, but when kept for some time they ac- 

 quire a brownish tinge ; I have obtained them 

 in flat colourless rhomboids, and in four-sided 

 right prisms with square bases. The prisms are 

 frequently six-sided from the truncature of two 



