1881).] On Selenic Acid and other Selenium Compounds. 17 



of seminormal soda, showing 97*67 per cent, of selenic acid. It was 

 next heated to 162°, and the strength of the residue taken : 0*7291 

 gram neutralised 19*72 c.c. of seminormal soda, equivalent to 97*85 

 per cent, of selenic acid, 



The same acid was then heated to 216°, and the residue obtained 

 was allowed to remain in the flask during the night. In the morning 

 it was found to be frozen into a crystalline mass so hard that it was 

 necessary to use a steel chisel in order to remove portions for examina- 

 tion. When dissolved in water and tested it was found to contain 

 some selenium dioxide. Trials were then made to ascertain if a lower 

 temperature would produce a similar result without decomposition of 

 the acid; 180° was found to be sufficient for the purpose, and the fol- 

 lowing course was finally adopted : — The acid, which had been con- 

 centrated on the water-bath as far as possible, was heated gradually 

 in the flask to 100°, and kept at that temperature so long as any acid 

 distilled over, the greater part of the water being thus removed. The 

 (J -tube was then disconnected, emptied, and refilled with stick 

 potash. The flask was next heated gradually to 180°, kept at that 

 temperature until no more acid distilled over, and then immediately 

 cooled. A still better arrangement was to use so little acid that it 

 was unnecessary to change the potash. The acid was heated gradu- 

 ally and continuously up to 180°, allowing bubbles to pass slowly 

 through it, as before described. When 180° was reached the potash 

 was watched, and as soon as it ceased to be acted upon the flask was 

 immediately cooled. An hour or less was generally found sufficient 

 time for a small quantity of acid. 



A specimen obtained in this way was found, when examined, to be 

 very free from selenium dioxide, a little of it diluted with water and 

 saturated with hydrogen sulphide, merely giving a faint yellow colora- 

 tion without any precipitate. Another portion was acidified with 

 hydrochloric acid and barium chloride added ; on boiling the filtrate 

 with stannous chloride it only became darkened in colour without any 

 precipitation of selenium. 0*6725 gram was taken to estimate the 

 strength : 18*54 c.c. of seminormal soda were required for neutralisa- 

 tion, equivalent to 99*73 per cent, of selenic acid. 0*724 gram of 

 another acid, prepared in a similar way, but which contained rather 

 more selenium dioxide than the last, required for neutralisation 

 19*94 c.c. of seminormal soda, equivalent to 99*64 per cent, of sele- 

 nic acid. A portion of a third acid, weighing 0*329 gram, was dis- 

 solved in water, barium chloride added, and also hydrochloric acid in 

 order to prevent any selenious acid from precipitating. The resulting 

 barium selenate weighed 0*6337 gram, equivalent to 99*75 per ceut. 

 of selenic acid. As has been already mentioned, the acid employed 

 contained an acid potassium selenate equivalent to 0*07 per cent, of 

 neutral sodium selenate found. Taking this into account, and calcu- 



vol. xlvi. c 



