Experiments on the Gas from Coal, 411 
tic and hydrogen gases, both of which may 
be traced in the aériform products of the ads 
vanced stages of distillation. As amatter of 
practice, it is certainly desirable that the azote 
existing in coal should enter into the composi- 
tion of a condensible fluid, rather than that 
it should escape in a gaseous state; for it is 
an impurity which, when once mingled with 
the combustible gas, cannot be removed by 
any known method, and must materially im- 
pair its illuminating power. That such an 
effect must result from its presence, may be 
inferred from the experiments of Sir H. Davy, 
who found that an explosive mixture of carbu- 
retted hydrogen and common air was deprived 
of its combustibility by being mixed with one 
sixth of its bulk of azotic gas.* 
On the Purification of Coal Gas. 
The chief impurities mingled with the gas 
from coal, which it is desirable and practi- 
cable to remove before applying it to use, 
are carbonic acid and sulphuretted hydrogen 
gases. The former is of little importance ; 
but the latter imparts to the coal gas, when 
* On the Safety Lamp, page 30, 
