298 M. Mitscherlich on a 7iew Acid of Selenium. 



to ascertain its precise proportion. Some acid which had 

 been heated higher than 280° C, subtracting the quantity of 

 selenious acid present, contained 84.21 of selenic acid, and 

 15.75 of water. If the oxygen in the water was to that in 

 the acid as 1 to 3, the solution should contain 12.38 per cent, 

 of water. It is certain that selenic acid is decomposed by heat 

 before parting with all the water which it contained ; and the 

 composition of the solution at 280 C. is analogous to that of 

 sulphuric acid which has been heated to 326." 



Selenic acid has a powerful affinity for water, and emits as 

 much heat in uniting with it as sulphuric acid does. Like sul- 

 phuric acid it is not decomposed by sulphuretted hydrogen, 

 and therefore this gas may be employed for decomposing the 

 seleniate of lead or copper. With muriatic acid the change is 

 peculiar ; for on boiling the mixture chlorine is disengaged, 

 and selenious acid is generated, so that the solution is capable 

 of dissolving gold or platinum hke aqua regia. Selenic acid 

 dissolves zinc and iron with disengagement of hydrogen, and 

 copper with formation of selenious acid. It dissolves gold also, 

 but not platinum. Sulphurous acid has no action on selenic 

 acid, whereas selenious acid is easily reduced by it. Conse- 

 quently, when it is wished to precipitate selenium from selenic 

 acid, it must be boiled with muriatic acid before sulphurous 

 acid is added. 



Selenic acid, in its affinity for alkaline bases, is little infe- 

 rior to sulphuric acid ; so much so, indeed, that the seleniate 

 of baryta cannot be completely decomposed by sulphuric acid. 

 It is therefore a very powerful acid. As its compounds are 

 isomorphous with those of sulphuric acid, and possess both 

 the same crystalline forms, and similar chemical properties, the 

 history of the sulphates, with a few slight but very interesting 

 modifications, is the same as that of the sulphates. 



The great number of crystallized compounds which this acid 

 produces, the different forms which they assume at different 

 temperatures, the beauty of the crystals admitting of precise 

 measurement, and the isomorphism of the seleniates with the sul- 

 phates, which with some chromates have supplied the most im- 

 portant facts towards that theory, induce M. Mitscherlich to 

 collect in an essay, that will be published in a few months, 

 the crystalline forms of the sulphates, seleniates, and chromates. 



