32 Prof. Sir C. A. Cameron and Mr. J. Macallan. [May 2, 



The green body formed by the action of selenium upon selenic acid 

 is, in all probability, a new oxide of selenium, having the composition 

 Se 2 3 , analogous to the sesquioxide of sulphur formed by the action 

 of sulphur upon sulphuric acid. 



The colour of the above compounds appears to be almost entirely 

 due to the element which is added on to the anhydride ; the sulphur 

 or selenium in the residual portion of the molecule has scarcely any 

 effect upon the colour. 



The green body SeS0 3 must be isomeric with the blue compound 

 SSe0 3 , and consequently the atoms of sulphur and selenium in each 

 of these bodies must occupy dissimilar positions in the molecule. 



The Possible Existence of Selenic Anhydride. 



A consideration of the foregoing facts leads to the conclusion that 

 selenic anhydride, which has not up to the present time been obtained, 

 is yet capable of existing in a free condition. Just as the sulphoxides 

 are direct addition compounds of sulphur, selenium, and tellurium 

 respectively, with sulphuric anhydride, so tbe selenoxides must be 

 regarded as addition compounds of the elements mentioned, with 

 selenic anhydride. It is difficult to conceive that bodies so constituted 

 are capable of existing free, and that the anhydride is unable to do 

 so. On the other hand, it is probable that it dissociates at a com- 

 paratively low temperature, but one that is higher than 75° — the 

 highest temperature at which the most stable of the selenoxides has 

 been as yet found to exist. Selenious anhydride is always obtained, 

 and not selenic, under similar conditions to those which produce 

 sulphuric anhydride, and involving a high temperature. 



We have examined the action of heat upon various selenates. 

 Those selected for the purpose were the selenates of antimony, 

 bismuth, platinum, lead, and silver, and also ferric and mercuric 

 selenates. In no instance was selenic anhydride obtained, but 

 selenious anhydride was evolved in all cases when the above selenates 

 were strongly heated. 



Yon Gerichten (< Liebig's Annalen,' vol. 168, 1873, p. 214) 

 endeavoured to form selenic anhydride in a similar manner to that 

 by which sulphuric anhydride is usually obtained — by leading a 

 mixture of oxygen and the vapour of selenious anhydride through 

 red-hot platinum sponge. He states that in one experiment a white 

 deposit was obtained which consisted partly of selenious anhydride, 

 but probably also contained selenic anhydride, since it dissolved in 

 water with a hissing sound, and the solution was found to contain 

 selenic acid. In subsequent experiments a decidedly negative result 

 was obtained, since only selenious anhydride was found in the 

 product. 



