1810.] extracted from Pyrites at Fahiun. 263 



tains twice as much oxygen as the bases, and its capacity of 

 saturation (that is to say, the quantity of oxygen in a base 

 which saturates 100 parts of the acid) is 14*37, 



Selenic acid gives salts with two different proportions of excess 

 of acid. In the first, the base is combined with twice as much 

 acid as in the neutral salts; and these compounds are very 

 soluble in water. There arc, however, some bases which I have 

 not been able to combine With an excess nf acid; viz. oxides ot 

 lead, silver, and copper, and protoxide of mercury. In this class, 

 which I shall call hisclcniatis, the acid contains four times as 

 much OXTgeh as the base. The alkaline biseleuiatcs act. as 

 acids, and we cannot produce any intermediate compounds 

 between those which are alkaline and those which are acid, if 



we except mixtures of the two in such proportions that the two 

 opposite properties arc destroyed; but evaporation causes the 

 salt with excess of acid to crystallize, while the other remains is 



the liquid with its apparent excess of alkali. In the salts with 

 the greatest degree of excess of acid, which 1 shall call i/inulri- 



selentates, the base appears to be combined with four times as 



much acid as in the neutral salts ; but 1 must mention that this 



conclusion is not founded on researches made on purpose, but 



is a deduction partly from direct experiments, and partly from 

 analog] . 1 have produced only ([iiadriseleniates w ith an alkaline 



base. 

 ( )n the other hand, selenic acid seldom gives salts with excess 



of base, and merely with those distinguished by their tendency 



to form such salts. For example, oxide of lead docs not form a 

 Bubseleniate when we digest its neutral seleniate with caustic 



potash ; but if we distil the neutral seleniate, it lets gQ aportion 

 of its acid, and leaves a suhsalt from which the acid cannot he 



expelled by heat. However, with the oxide of copper, wo obtain 



a suhsalt even in the humid way. I have not examined the 

 ratio between the acid and base in these suhsalts. 



The selenrates arc usually decomposed h\ a degree of heat 



far from considerable. This decomposition is occasioned by 



foreign combustible bodies which almost always mix with them, 



especially ifthey be passed through the filter. The combustible 



bodj decomposes a portion of the acidj but. the selenium 



remains in combination with the base. An alkaline seleniate 

 thus treat <1 dissolves in water with a red colour ; and an earthy 

 seleniate Leaves the selenium in red (locks, when we dissolve it 



m an acid. If this phenomenon docs not appear before heating 

 the seleniate, we mix it with a little nitrate, the acid of which is 



decomposed in preference. 



If we mix an alkaline or alkaline earthy seleniate with char- 

 COal in powder, and then heat it to redness, the selenic acid is 



decomposed without detonation,. Carbonic aoid and oxide ol 

 < arbon are disengaged ; a small quantity of selenium sublimes ;_ 

 but the greatest pari remains in combination with the base. •' 



