1819.] extracted from Pyrites at Fahlun. 407 
acid was sublimed ; and the mercury was driven off by exposing 
the seleniate of potash to a red heat. 
I have found that seleniate of potash containing sulphate of 
potash and oxide of mercury, when decomposed by an excess of 
sal ammoniac, yields pure selenium; for the sulphate of potash 
is not decomposed, and the oxide of mercury driving off a por- 
tion of the base of the sal ammoniac forms a double salt easily 
soluble in water ; and the mercury is not reduced by the hydrogen 
of the alkali. But the selenium thus obtained often contains 
oxide of tin mechanically mixed, from which it may be purified 
by distillation. If, on the other hand, we saturate with ammo- 
nia selenic acid containing a mixture of sulphuric acid, and 
distil the resulting salt without the addition of fixed alkali, we 
obtain a selenium which contains a great deal of sulphur. 
Another method, the object of which is to spare a part of the 
acids, is to distil the precipitate obtained by means of sulphur- 
etted hydrogen in a glass retort. There is disengaged at first a 
great deal of sulphuretted hydrogen gas, then sulphur comes 
over containing a little selenium, and it becomes more and more 
impregnated with it as the process goes on. It has the colour of 
lead, a metallic lustre, and continues for a long time elastic. 
Towards the end, we obtain in the upper part of the retort a 
metallic substance crystallized in aconfused manner. It is sele- 
niuret of mercury proceeding from the decomposition of sulphuret 
of mercury by selenium at a high temperature. At the bottom 
of the retort remains a mixture of sulphuret of copper and sul- 
phuret of tin. 
Before discovering the method of reducing selenium by means’ 
of sal ammoniac, I tried to accomplish it by means of iron or 
zinc, plunging these metals into acid solutions of selenium, But 
this reduction is slow, incomplete, and does not yield a pure 
product. It was for this reason that I abandoned it. 
3. Selenium in a State of Purity. 
When selenium, after being fused, becomes solid, its surface 
assumes a metallic brilliancy, of a very deep brown colour, 
resembling polished hematites. Its fracture is conchoidal, 
vitreous, of the colour of lead, and perfectly metallic. If melted 
selenium be exposed for some time to heat, so as to cool very 
slowly, its surface becomes rough and granular, and of the 
colour of lead. The fracture is granular, dull, and looks exactly 
like a piece of metallic cobalt. If it be again fused and cooled 
rapidly, its surface becomes smooth, and its fracture vitreous as 
at first. Selenium has very little tendency to assume a crystal- 
lized form. When slowly separated from a solution of hydro- 
seleniuret of ammonia, it forms on the liquid a pellicle, the 
upper surface of which has a pale leaden colour, and appears 
smooth ; while the surface next the liquid has’a darker colour, 
and appears covered with small polished points. Under the 
