124 
best gas from coal about 550. Both 
these numbers ate, however, much above 
the average of the specific gravity of the 
two descriptions of gas usually supplied 
to the public. In the manufacture of 
oil-gas, it is not only necessary to have 
the best fish oil in order to produce gas 
of the specific gravity 950 or 960; but 
if the process be not carefully super- 
intended, the gas becomes partially de- 
composed and the carbon deposited ; 
the remaining gas being considerably 
lighter, and consequently less valuable 
for the purposes of illumination. If oil- 
gas be exposed to a white heat it will 
deposit half its carbon (and _conse- 
quently half its value), while the vo- 
lume of the hydrogen will at the same 
time be expanded: for the combination 
of carbon with hydrogen gas appears, 
in all cases, to condense the volume of 
the latter in proportion to the quantity 
of carbon.in the compound gas. 
. Oil gas, or heavy carburetted hydro- 
gen, is supposed to consist of equal 
volumes of hydrogen and carbon, the 
weight of the carbon being about six 
times greater than that of the hydrogen, 
Weare therefore justified in concluding 
that the actual value of oil-gas for illu- 
mination is in the proportion of six to 
seven of its specific gravity ; provided we 
allow the hydrogen to be of no yalue. 
But as the hydrogen in oil-gas is in a 
state of considerable condensation, we 
may assume it as furnishing at least one 
seventh part-of the light produced in 
combustion ; and the carbon, as fur- 
nishing the remaining six parts. And 
by comparing this estimate with the 
known specific gravity of oil (or olefiant) 
gas, we should have in numbers.136 (by 
weight) for the hydrogen; and 136+6 
= 816 for the carbon; or 816 carbon + 
136 hydrogen = 952 for the density of 
the compound gas. 
It has not yet, I believe, been deter- 
mined by any chemist what is the ut- 
most limit of combination ; or we should 
say, the point of saturation between 
hydrogen and carbon in the state of 
gas. But the combination of these two 
substances in a solid state may afford us 
a pretty good estimate in this case. 
Thus the density of the inflammable 
gas from wax or spermaceti is found 
to be from .980 to .1000, or nearly 
equal to that of common air ; and it is 
obvious, that the nearer the weight of 
oil-gas approximates to this standard, 
the greater will be its value for the pur- 
poses of illumination. 
By the same standard also we may 
Comparative Value of Coal and Ol Gas. 
[Serr. 1, 
estimate the value of coal-gas, or sub- 
carburetted hydrogen. 
The quality as well as the quantity, 
of gas obtained from coal, varies even 
in a greater degree than that from oil. 
The crude gas, as it comes over from 
the retorts, is a combination of heavy 
carburetted-hydrogen, light carburetted- 
hydrogen, carbonic-oxide, tar vapour, 
sulphur, ammonia, &c.: the latter sub- 
stances being almost entirely deposited 
subsequently, by the process of purifi- 
cation. Some kinds of coal yield a 
much richer burning gas than others, 
but the average density of purified coal- 
gas is considerably below 500 specific 
gravity. _Coal-gas however, like oil-gas, 
is liable to be decompounded, and con- 
sequently lowered in value when pro- 
duced under too high a temperature. 
But this matter being perfectly well un- 
derstood by the managers of gas works, 
it is not necessary to dwell on the sub- 
ject here. 
In order therefore to estimate the 
comparative value of oil-gas and coal-gas 
to the consumer, I shall suppose the 
average density of the coal-gas used in 
the metropolis to be about .450 (which 
cannot be far distant from the fact) ; 
and that the gas companies charge the 
public at the rate of about Is. 6d. for 
every hundred cubic feet of gas. Now 
if we estimate the value of inflammable 
gas by its density (which appears to be 
the only accurate mode of estimate) it 
will be obvious that, if coal-gas, of .450 
spec. grav. be worth ls. 6d. per 100 feet, 
oil-gas of .950 spec. grav. will be some- 
thing more than double, or about 3s. 2d. 
per 100 feet. But the manufacturers of 
oil-gas, in all parts of the kingdom 
where it has yet been introduced, have, 
I believe, charged their consumers at 
the rate of 4s. 6d. to 5s. per 100 feet. 
And, in order to justify this charge, 
they state that oil-gas (without giving 
any density, as data) is more durable 
than coal-gas, in the ratio of three and 
a-half to one, under similar circum- 
stances. The question as to value, is 
howeyer by no means satisfactorily de- 
termined from a comparison with the 
eye between the lights emitted by two 
gas-burners. It is undoubtedly true, 
that from two. jets of gas of equal dia- 
meter the jet of oil-gas will give the 
most vivid and brilliant light; but if we 
diminish. the volume of the flame of oil- 
gas till the quantity of light given off in 
the area of a room shall be as nearly 
equal as possible to the eye, the dura- 
bility of the oil-gas will not exceed that 
of 
