1819.] extracted from Pyrites at Fahlun. 409 
flowers, as happens to sulphur in the same circumstances. The 
smell of horseradish is not perceived till the heat becomes great 
enough to occasion oxidation. 
Selenium is not a good conductor of heat. We can easily 
hold it between the fingers and melt it at the distance of one or 
two lines from the fingers without perceiving that it becomes 
hot. It is also a non-conductor of electricity. I put a piece of 
selenium an inch in length and a line in diameter in contact by 
one end with the conductor of an electrical machine, and by the 
other with a chain which was to conduct the electricity into the 
earth. The conductor always gave sparks at the distance of 
three quarters of an inch when another conducting body 
approached it. When I attempted to discharge a Leyden phial 
by the same piece of selenium, the discharge took place with a 
jong hissing noise, and a good deal of the electricity remained 
in the jar. Ifthe charge was very strong, the electricity passed 
in the form of a spark along the surface of the selenium ; but if 
there was another shorter road, the spark did not follow that 
surface, as would have been the case if the selenium had been a 
conductor, as we observe with water, gilt paper, &c. But on 
the other hand I have not been able to render it electric by fric- 
tion, at least to a degree that I could appreciate ; so that sele- 
nlum cannot be reckoned among idioelectrics. 
It is not hard: the knife scratches it easily. It is brittle like 
glass, and is easily reduced to powder. 
I have found its specific gravity between 4:3 and 4:32. It is 
difficult to take the specitic gravity of it with certainty ; because 
small cavities often occur in the middle of its mass. Slow cool- 
ing, which gives it a granular fracture, did not appear to me to 
alter its specific gravity. 
4. Selenium and Oxygen. 
The affinity of selenium for oxygen is not very great. If we 
heat it in the air without touching it with a burning body, it is 
usually volatilized without alteration ; but if itis touched by flame 
it gives to its edges a fine sky-blue colour, and is volatilized 
with a strong smell of horseradish. The odorous substance is 
a gaseous oxide of selenium, which, however, I have not been 
able to obtain in an isolated state, and without being mixed with 
atmospheric air. This oxide does not appear to possess the 
properties of combining with acids, and of course belongs to 
the same class of oxides as carbonic oxide. To these I have 
iven the name of suboxides. 
Oxide of Selenium.—If we heat selenium in a close phial filled 
with common air till the greatest part of it is evaporated, the 
air of the phial acquires the odour of oxide of selenium ina very 
high degree. If we wash the air with pure water, the liquid 
acquires the odour of the gas; but as there are always formed 
traces of seleni¢ acid, this water acquires the property of redden- 
