422 Experiments on the Gas from Coal. 
detonation is attended with a great saving of 
time. But on all occasions where only few 
experiments are required on gases of great 
combustibility, I prefer slow combustion, both 
on account of greater safety to the appa- 
ratus, and, from the quantities that may be 
consumed, of greater accuracy also. When - 
rapid combustion is practised, I believe that, 
on the whole, more accurate results are gained 
by firing the gas at one operation properly 
conducted, than at two. The latter method 
seems to have been preferred by M. Berthollet, 
but so far as my experience goes, it is more 
apt to precipitate charcoal from the gas. 
To burn each measure of the early and 
more combustible products of gas, I employed 
from 3 to 4 measures or upwards, of oxygen 
‘gas, the degree of purity of which had been 
ascertained. ‘The volume being noted after 
firing, and again after agitating the residue 
with liquid potash, the last diminution shewed 
the quantity of carbonic acid. The gas left 
by potash was next analyzed by combustion 
with a due proportion of pure hydrogen,* 
which shewed how much of the residue was 
oxygen, and how much azotic gas. If more 
* The method of doing this is given in my Elements of 
Chemistry. Vol. I. chap. v. Sect. vi. 
