analogous Selenium Substitution Product of the same. 407 



which was covered with a porcelain crucible-lid. The selenium 

 absorbs the anhydride somewhat more slowly than sulphur 

 does. A fine glass rod is used for determining whether any 

 little particles of selenium remain unacted upon. When the 

 whole of the selenium is combined with the anhydride, the 

 excess of the latter is poured off, the last particles being re- 

 moved by the application of a gentle heat in the manner 

 already described. Decomposition of the compound may be 

 caused during the heating ; but inasmuch as the selenium is 

 more stable than the sulphur compound, this source of error 

 is not to be so greatly feared. 



The substance which has been thus prepared forms a solid 

 crust, which strongly adheres to the glass. It is removed by 

 means of a rough glass rod, and quickly brought into a carefully 

 enclosed flask, or better, into a glass tube, which is then sealed. 



Properties. 

 In the moment of its preparation this compound is tar-like 

 and viscid. Upon the removal of excess of anhydride it forms 

 a solid crust, which, under the magnifying-lens, is seen to 

 consist of a mass of minute prismatic crystals. In the com- 

 pact form this compound is of a dirty green colour ; in the 

 state of powder, as it is obtained by rubbing together the 

 crust with a glass rod (of course, in the absence of moisture), 

 its colour is yellow. The compound is more stable than 

 dithionic oxide ; it may be preserved in sealed glass tubes, 

 and may even be gently heated without decomposition. Like 

 the sulphur compound, it cannot be fused after the first solidi- 

 fication ; at a high temperature it is decomposed, with the 

 evolution of sulphurous and selenious acids, and deposition of 

 selenium. This compound is insoluble in pure sulphuric an- 

 hydride ; it is, however, soluble in fuming, in English, and in 

 somewhat dilute sulphuric acid. The addition of water causes 

 a violent reaction, resulting in the deposition of selenium in 

 the form of a coherent mass, and in the production of sulphuric, 

 sulphurous, and selenious acids, which remain in solution. 



Composition. 

 The quantities of sulphur and of selenium in the compound 

 were estimated by decomposing with water a weighed quantity 

 contained in a little stoppered flask, and placed within a larger 

 flask, also stoppered. When the action was complete, the 

 precipitated selenium was collected on a weighed filter; and to 

 this was added the small quantity of selenium which was 

 thrown down from the filtrate on the addition of ammonium 

 sulphite. In the liquid thus freed from selenium, sulphurous 

 acid was oxidized to sulphuric by means of a little pure fuming 

 nitric acid, and barium chloride was added so long as a preci- 

 pitate was obtained. The following numbers were obtained;-— 



