PRESENT STATE OF THE CHEMISTRY OF GAS-LIGHTING. 283 
1. The combustible non-illuminating constituents. 
2. The illuminating hydrocarbons. 
3. The impurities. 
1 * ^ ie Combustible Non-Illuminating Constituents of Coal Gas. 
, These form a very large proportion of its bulk—as from 30 to nearly GO per cent. They 
are y rogen and carbonic oxide; and their sole function seems to be the purveying or 
carrying the illuminating hydrocarbons. 
(a) Hidrogen (H) exists in coal gas to the extent of from 12 to nearly 50 per cent. 
is in giea est abundance in poor gas of low illuminating power, and it no doubt 
comes m grea - pait from the decomposition of the richer hydrocarbons by the high tem¬ 
perature of the retorts. It is also produced by the moisture of the coal giving up its 
ox ygeii o le le lot carbon, and setting free its hydrogen. The gas is singularly inert 
it has no colour, no taste, no odour, and no action on the human body. It is the 
lightest body known, its specific gravity being 0-0692, atmospheric air being 1; and 100 
cubic inches of it weigh only 2-15 grains, the same quantity of air weighing 31 grains. 
It burns without any light; and, as you perceive when I hold a cold glass over the 
iiame, the product of its combustion is water. One cubic foot of hydrogen requires half 
a cubic foot of oxygen,, or cubic feet of atmospheric air, for combustion. The tem- 
peiatuie of the flame is very high, about 5898° Fahr.; and a cubic foot of the gas, in 
burning, wdl raise 5220 ounces of water, or 16,250 cubic feet of air, 1° Fahr. Lastly, 
it is but slightly absorbed by water—100 volumes of water taking up about 1-93 of hy¬ 
drogen gas, and it is not condensable by cold or pressure. 
(b) Carbonic. Oxide (CO) is. a compound of one proportion of carbon and one of 
ox ygen. }’ weight, therefore, it consists of*6 parts of carbon and 8 of oxygen ; and by 
volume, of half a volume of carbon vapour and half a volume of oxygen, united without 
any condensation. It is not a large constituent of coal gas, the proportions ranging from 
about o to 16 per cent.; but it forms a considerable part—about 34 per cent.—of the 
gas made by the decomposition of steam by red-hot carbon. This, indeed, is the chief 
source of it in common coal gas. Like hydrogen, it is colourless and odourless; but, un¬ 
like it, it is a deadly poison. I have ascertained that 2 per cent, of it in air will kill 
nrds almost instantly ; .and., according to Leblanc and Dumas, an atmosphere contain¬ 
ing 1 per cent, of it will kill a small dog in a minute and a half. It is a little lighter 
than atmospheric air m the proportion of 0-967 to 1; and 100 cubic inches of it weigh 
nearly 30 grains. It burns with a pale bluish and somewhat opaque flame; and, as you 
here see, it forms nothing but carbonic acid. The flame cannot be got from the small 
jet over the governor. I am obliged to burn the gas from a very large iet over the 
pneumatic trough, and therefore I am not able to show you the size of its flame in com¬ 
parison with coal gas. In the act of burning, 1 cubic foot of carbonic oxide consumes 
half a cubic foot of oxygen, or 2\ cubic feet of air; and the heat of the flame is very 
nearly as great as that of hydrogen, it being 5508° Fahr. A cubic foot of it will raise 
the temperature of 5400 ounces of water, or 16,500 cubic feet of air, 1° Fahr It is but 
slightly absorbed by water—100 volumes absorbing 2-43 volumes—but it is very freely 
absorbed by a strong solution of subchloride of copper; and this is the agent used for 
its detection in coal gas. 
2. The Illuminating Constituents (Hydrocarbons) of Coal Gas. 
These are all compounds of carbon and hydrogen; hence th^ir name, hydrocarbons. 
And.as, in the same volume of the gas, there are very different proportions of carbon 
the illuminating power of these several constituents varies considerably. In all cases 
* e light which they evolve in the act of burning is due to their decomposition by the 
heat of the flame, and to the suspension of the liberated carbon for a definite time in an 
ignited state. 
The following are the principal members of this group, beginning with the least illu¬ 
minating:— 
Marsh Gas, C 2 H 4 ; Olefiant Gas, or Ethylene, C 4 H 4 ; Propylene, C 6 H G ; Butylene, 
8 , \’xT Ce ^ e ^°’ ’ Benzol, C^Hg; Naphthaline, C. 1() .H 8 , and perhaps some others. 
(a) Marsh Gas is also called Pit Gas, Fire-damp, Light Carburetted Hvdrogen, and 
Hydride of Methyl (Co H 4 ). A volume of the gas contains half a volume of carbon 
vapour, and two volumes of hydrogen. It is a large constituent of the gas evolved from 
u 2 
