516 SELENIOUS ACID. 



with oxygen, forming a gaseous oxide of selenium* and like arsenic 

 it is odorous only while combining with oxygen at a high temperature. 



(b.) Formed best by heating selenium in a close glass vessel, with a 

 limited quantity of air, which is to be washed to remove the selenic 

 acid, a little of which is formed at the same time ; the water acquires 

 the smell of the gas, and feebly reddens litmus. 



The oxide of selenium is only sparingly soluble in water, and does 

 not combine with alkalies. Its composition has not been ascertained, 

 but it is supposed to be one proportion of each of the constituents. 



SELENIOUS ACID. 



1. PROPERTIES. 



(a.) Selenium is combustible. Heated in a flask filled with oxy- 

 gen gas, selenium evaporates with the odor of oxide of selenium,, but 

 without inflaming, and exactly as it would do in common air ; but 

 if heated in a glass ball of an inch in diameter, and supplied with 

 oxygen gas at the moment of ebullition, it burns with a feeble flame, 

 white towards the base, and green, or bluish green on the edges : the 

 selenium is completely consumed, oxygen gas is absorbed, and the 

 remaining ga has the odor of oxide of selenium. The product is a 

 sublimate of selenious acid. 



(b.) Hot nitric acid dissolves selenium, and forms on cooling large 

 prismatic crystals of selenic acid, longitudinally striated, and resem- 

 bling almost exactly those of nitrate of potash. 



(c.) This acid is still better prepared by the aid of nitro-muriatic 

 acid. A white residuum is left on evaporation, and by an increased 

 heat the selenious acid sublimes, and is condensed in the colder part 

 of the apparatus in very longf needles of four sides. The vapor of 

 the acid has a deep yellow color much resembling that of chlorine, 

 but not so deep as that of the vapor of the selenium itself. 



(d.) Selenious acid has a peculiar lustre which it quickly loses on 

 being exposed to the air ; the crystals adhere and it gains weight so 

 fast that it is difficult to weigh it accurately. Taste acid, leaving a 

 slightly burning sensation. 



(e.) Readily soluble in cold and almost without limit in hot water, 

 from which, by rapid cooling, it crystallizes in grains, and more slow- 

 ly in prisms, and spontaneously in acicular radiated groups. Very 

 soluble in alcohol and giving with that fluid by distillation an etherial 

 odor, intermediate between that of nitre and sulphuric ether. 



(f.) Sulphuric acid, selenic acid, and alcohol in mixture produce 

 by distillation, a most insupportable odor. Decomposition. Easily af- 



* Which Berzelius thinks analogous to the oxide of carbon, although ho has not 

 been able to isolate and shew it separately. 



t In a large retort they are sometimes two inches or more long. 



