1889."] On Selenic Acid and other Selenium Compounds. 



31 



investigated the reaction, and in 1875 Weber (Poggendorff's 

 ' Annalen,' vol. 156, p. 545) isolated the green compound, and found 

 it to be a sulphoxide of selenium, SeS0 3 . The colour given by 

 selenium with sulphuric acid is a dark green, not so bright as that 

 which it produces with selenic acid. 



The Action of Tellurium upon Selenic Acid. 



Like selenium, tellurium reacts upon anhydrous selenic acid in the 

 cold, but with production of a purple-red colour. Even when the acid 

 is not perfectly anhydrous, it will still give a coloration in the cold. 

 The compound thus formed is probably very unstable, since the colour 

 disappears at so low a temperature as 19°, the selenic acid being partly 

 reduced to selenious acid. If water is added when the colour has 

 developed, tellurium is thrown down in sooty flakes, and exposure to 

 moist air produces the same effect. 



Tellurium reacts in a similar manner with strong sulphuric acid, 

 with development of a red colour, and re -precipitation on addition of 

 water. This was first observed by Muller von Reichenstein. In 1789 the 

 same reaction was investigated by Klaproth, and afterwards by Magnus 

 and Fischer. The red compound was isolated by Weber in 1882 

 (' Journal fur Praktische Chemie,' vol. 25, p. 218), and soon afterwards 

 independently by Divers and Shimose (' Journal of the Chemical 

 Society (Transactions),' vol. 43, p. 319), and was ascertained to be 

 tellurium sulphoxide, TeS0 3 . 



Comparison of Sulphoxides with Selenoxides. 



The coloured compounds first formed by the action of sulphur, 

 selenium, and tellurium, upon sulphuric acid were subsequently 

 produced by the action of the same bodies upon sulphuric anhydride, 

 and were isolated by forming them in the latter manner, and then 

 removing the excess of anhydride. As has been already mentioned, 

 they were found to be sulphoxides having the composition respec- 

 tively SS0 3 , SeS0 3 , and TeS0 3 . Considering the similarity of the 

 reactions which produce them, and of the decompositions to which 

 they are liable, it is very probable that the coloured bodies formed 

 with selenic acid are selenoxides, analogous to the sulphoxides already 

 known, the sulphur in the latter being replaced by selenium. A com- 

 parison of the formulas of the two series of bodies as represented in 

 the following table, leads to some interesting considerations : — 



Colour of 



compound. Sulphoxides. Selenoxides. 



Blue SS0 3 SSe0 3 



Green SeS0 3 SeSe0 3 



Red TeS0 3 TeSe0 3 



