3oo Dr. Henry’s Description of an Apparatus for the 
which was produced. The specific gravity of the coal gas 
appears to afford a measure of its fitness for illumination, suf- 
ficiently accurate for practical uses ; but does not bear an exact 
correspondence to the chemical properties of the gas, as as- 
certained by combustion. It may be remarked, also by com- 
paring the two last columns of the second table, that the car- 
bonic acid produced does not always bear the same propor- 
tion to the oxygen expended. Thus the first product of gas 
from cannel coal combines with 234 measures of oxygen gas ; 
and gives 139.7 of carbonic acid. But the gas from coal tar, 
with only an equal consumption of oxygen, yields 150 mea- 
sures of carbonic acid. 
5. The aeriform product of coal does not precisely answer 
to the characters of any one of the combustible gases, with 
which we are acquainted. The first product, however, of the 
distillation of common pit coal, after being washed with potash, 
approaches very nearly in its properties to carbureted hydro- 
gen gas. The gases, which surpass this in specific gravity, 
are mixtures of carburetted hydrogen with olefiant gas, and 
perhaps a small proportion of carbonic oxide. The lighter 
gases, in addition to carbureted hydrogen, probably contain a 
variable proportion of hydrogen gas and a small quantity of 
carbonic oxide. The extreme levity of some of the products, 
especially of the gas from Merthyr coal, cannot be explained 
on any other supposition. 
6 . The products of the combustion of a cubic foot of coal- 
gas, of medium quality, viz. of the specific gravity 6 22, (such 
as the first products from Newcastle on Tyne coal ) may be 
stated as follows: 
