14,2 Dr. Henry on the aeriform compounds 
last number is the weight of the atom of charcoal, as deduced 
from the constitution of olefiant gas. It is true, that this 
determination a little exceeds that which is derived from the 
composition of carbonic acid (viz. 5 65 ), the atom of oxygen 
being taken at 7-5. But if 8 be the true number for oxygen, 
which now seems to be most probable both from experiment 
and analogy, we shall then find an exact coincidence be- 
tween the relative weight of the atom of charcoal, as deduced 
from olefiant gas, and as determined from carbonic acid. 
Perhaps the true specific gravity of hydrogen gas, on which 
depend the relative weights of the atoms of hydrogen and 
oxygen, may be fully as correctly ascertained from the com- 
position of carburetted hydrogen, as by direct attempts to 
weigh so light a fluid. Now, as the hydrogen in 100 cubic 
inches of hydro-carburet weighs only 4*2 6 grains, and is 
equivalent to 200 cubic inches of hydrogen gas, we have 2*13 
grains for the weight of 100 cubic inches of hydrogen gas, 
from which may be deduced *0698 for its specific gravity, 
that of air being 1. And if the specific gravity of oxygen 
gas be 1*111, it will be found that the two volumes of hy- 
drogen, required to saturate one volume of oxygen gas, have 
as nearly as possible the relative weight of 1 to 8. 
Were any additional argument necessary to establish the 
existence of carburetted hydrogen as a distinct species, it 
might be derived from the action of water on that gas, which, 
besides being absorbable in a constant proportion, admits of 
being expelled again by the application of heat, not other- 
wise changed than in having acquired a small quantity 
of those gases which are always present in water, and of 
