1889.] On Selenic Acid and other Selenium Compounds. 



15 



already been fully pointed out by Marignac (' Annales de Chimie,' 

 vol. 39, 1853, p. 184), the discordant results arrived at by various 

 observers being probably due to the different conditions under which 

 they worked. The latter precaution is particularly necessary, as we 

 have observed that sulphuric acid occupies less volume in the solid 

 than in the liquid state. 



The temperature of the acid was then gradually lowered. Its 

 viscosity increased as it became colder, until at a temperature of 

 — 515° C. it was as thick as soft pitch — the tube might be inverted 

 without the acid flowing, and a glass rod could be moved in it only 

 with great difficulty. Still it did not freeze, until after vigorous 

 stirring maintained for a couple of minutes, a crystal appeared, and 

 then the entire mass almost instantaneously crystallised, the tem- 

 perature rapidly rising. When removed to a warm room, the crystals 

 were rather permanent, and when nearly melted, recrystallisafcion 

 could easily be induced by exposure to moderate cold so long as any 

 crystal remained, showing that previous to freezing the acid had been 

 in a more or less superfused condition. It was found to be impossible, 

 however, to separate the crystals by draining the liquid portion, as 

 the latter was so viscous that it carried the crystals with it. Under 

 the microscope they were seen to be long prisms. 



An attempt was next made to examine the conditions under which 

 selenic acid becomes strengthened in a vacuum, with the object of 

 obtaining, if possible, a more concentrated acid ; and the following 

 arrangement was made for the purpose : — The glass receiver of an 

 air-pump was fitted tightly at its neck with an india-rubber cork, 

 through which passed a bent tube, connected horizontally with 

 another and wider tube containing solid potash. This was connected 

 with a wide [J -tube, filled with pieces of potash about half an inch 

 long, and kept cool by immersion in a beaker of water. Connexion 

 was then made with a small stout flask containing the selenic acid, and 

 varying in size from 100 to 250 cubic centimetres as required, The 

 potash-tube next the receiver was intended for the purpose of pre- 

 venting acid fumes from injuring the air-pump, and for the same reason 

 a vessel of potash was placed within the receiver. The air-pump 

 employed gave a very good vacuum ; when all the connexions were 

 made, and the pump exhausted, there was often scarcely any appre- 

 ciable difference in the levels of the columns of mercury in the gauge. 

 For temperatures up to 100° the flask containing the acid was heated 

 in a beaker of water ; for higher temperatures oil was used. Soon after 

 commencing the experiments it was found necessary to make an 

 arrangement for the purpose of stirring up the viscous acid and 

 exposing fresh surfaces to the vacuum, and the following plan was 

 devised : — A test-tube, which fitted easily the neck of the flask, was 

 shortened by removing evenly a portion of the open end. It was 



