SELENIC ACID. 517 



fected by all bodies having a strong affinity for oxygen, as sulphurous 

 and phosphorous acids, alkaline sulphites, and sulphuretted hydrogen, 

 and metallic zinc,* by all of which it is precipitated. Zinc throws 

 it down in the form of red, brown, or blackish flakes : sulphuretted 

 hydrogen in an orange precipitate, fusible a little above 212, sub- 

 limed in close vessels, burning in the air and producing selenic and 

 sulphurous acids. 



(g.) Selenium is soluble in oils; it unites with the metals usually with 

 ignition, forming seleniurets commonly of a gray color and metallic 

 lustre. The seleniuret of potassium is soluble in water with efferves- 

 cence. The acids disengage from it seleniuretted hydrogen, whose 

 odor is like that of sulphuretted hydrogen but excessively offensive. 



This gas is soluble in water, combines with the alkalies, and pre- 

 cipitates metallic salts of a dark color. 



2. EQUIVALENT NUMBER. Berzelius from his investigations con- 

 cludes that selenious acid consists of 



Selenium, 71.261 100.00 



Oxygen, 28.739 40.33 



If it is composed of one proportion of base and two of oxygen, the 

 equivalent number of selenium will be 40+2 oxygen 16=56ybr tht 

 equivalent of selenic acid. 



SELENIC ACID. 



The acid just described has been hitherto called by this name, but 

 another acid has been discovered containing an additional equivalent 

 of oxygen, and which is therefore called selenic acid. 



1. PREPARATION. Omitting the tedious process upon the selenitic 

 ores,f we may describe that which commences with the preceding 

 acid, the selenious. 



(a.) It is neutralized by soda, and by fusion with nitre or with ni- 

 trate of soda it is converted into seleniate of soda and crystallized. 



(b.) This seleniate is decomposed by nitrate of lead, which gives 

 an insoluble seleniate. 



(c.) This is decomposed by a stream of sulphuretted hydrogen, 

 which precipitates the lead as a sulphuret and liberates, without de- 

 composing the selenic acid ; the excess of sulphuretted hydrogen be- 

 ing expelled by heat, the selenic acid remains diluted with water. 



2. PROPERTIES. 



(a.) Colorless; not decomposed below 576 Fahr. but above that 

 emits oxygen and becomes selenious acid. 



(b.) Sp. gr. When concentrated at 329, it is 2.524 ; if at 512, 

 it is 2.60 ; and if at 545, it is 2.625 ; but a little selenious acid is 



Mixed with muriatic acid. 



Edin. Jour, of Science, No. XVI, p. 294, and Turner, 2d ed. p. 350. 



