296 M. Mitscherlich on a New Acid of' Selenium. 



is done by heating a mixture of the seleniate of soda and sal- 

 ammoniac, when mutual decomposition ensues, and selenium, 

 nitrogen, and water are evolved. The selenium thus obtained 

 is quite free from sulphur. It is then dissolved in an excess 

 of nitric acid, and any trace of sulphuric acid may be detect- 

 ed and removed by muriate of baryta, which, however, does 

 not render it turbid. The acid solution is then neutralized by 

 carbonate of soda, the selenite of soda converted into the 

 seleniate by fusion with nitre in a crucible of porcelain, and 

 the nitrate of soda removed by crystallization in the man- 

 ner above described. The pure seleniate is dissolved in water, 

 and obtained in crystals by spontaneous evaporation. 



To procure the acid in a free state, the seleniate of soda is 

 decomposed by the nitrate of lead. The seleniate of lead, 

 which is as insoluble as the sulphate, after being well washed, 

 is treated by a current of sulphuretted hydrogen gas, which 

 does not decompose selenic acid. The excess of sulphuretted 

 hydrogen is driven off by heat, and pure selenic acid remains 

 diluted with water. The absence of fixed substances may be 

 proved by its being volatilized by heat without residue ; and 

 if free from sulphuric acid, it gives no precipitate with the mu- 

 riate of baryta after being boiled with muriatic acid. Any 

 nitric acid which may be present is expelled by concentrating 

 the solution. 



Composition of Selenic Acid and the Seleniates.^-Since the 

 neutral salts of selenic acid are isomorphous with the sulphates, 

 the composition of selenic acid and seleniates may be expected 

 to obey the laws of isomorphism. Consequently, selenic acid 

 should contain one-half more oxygen than selenious acid for 

 the same quantity of selenium ; and the oxygen contained in 

 the base of the seleniates ought to be a third of the oxygen of 

 the acid. Experiment fully confirms this supposition. 



Of fused seleniate of potash, 2.6545 yielded 1.7655 of the 

 chloride of potassium, equivalent to 1.117 of potash, and 3.34J35 

 of the seleniate of baryta, indicating 1.5315 of selenic acid ; 

 because 1.812 of baryta is equivalent to 1.117 of potash. 

 Accordingly, 100 parts of the seleniate of potash consist of 



Potash, 42.16 of which the oxygen is = 7.15 



Selenic acid, 57.84 do. = 21.79 



