329 
PRESENT STATE OF THE CHEMISTRY OF GAS-LIGHTING. 
put into the potash solution, which is still in contact with the gas, a little pyrogallic 
acu, and again agitate. If oxygen be present, the solution will acquire a brown colour, 
and the further diminution of volume is due to the absorption of oxygen. I ought to 
say that the gas is heavier than air in the proportion of 1 to 1-105G ; and 100 volumes 
of water will absorb about 2-99 volumes of it at common temperatures. 
(c) Nitrogen (N) is another impurity derived from the air by the action of the ex¬ 
hauster. The gas has no very marked influence on the luminosity of the flame, beyond 
this—that, in expanding under the influence of the temperature of the flame, it cools if 
somewhat by abstracting heat; and another objection to it is its power of forming nitric 
acul when it burns with the gas. There is no ready test for the discovery of this gas. 
It is a little lighter than air—its specific gravity being 0-9713; and it is but feebly solu¬ 
ble in water 100 volumes taking up about 1*48 volume of the gas. 
(d) Ammonia (Nil.,).—This is a product of the carbonization of the coal, and it may 
exist m the gas in a free state, or combined with carbonic acid. A volume of the gas 
consists of half a volume of nitrogen and one and a half volume of hydrogen. Its spe¬ 
cific gravity is 0-5896, and its objectionable properties are that it attacks copper and 
brass fittings, and that in burning with the gas it forms nitric acid; and that it is a pur¬ 
veyor of naphthalin, and other heavy and fetid hydrocarbons in coal gas. The gas is 
readily absorbed by water; at common temperatures water will take up about 727 times 
its volume of the gas. It is also absorbed by dilute acids, and by many neutral salts. 
There is, therefore, no difficulty in removing it from gas. The test for it is moistened 
turmeric paper, which becomes red in less than a minute when the gas contains no more 
than 1 grain of ammonia in 100 cubic feet. 
(e) Bisulphide of Carbon (CSg).'—This compound is almost invariably present in 
coal gas, and it is no doubt formed in the latter stages of the distillation when the tem¬ 
perature is high, and when the sulphur liberated from the pyrites comes into contact 
with the incandescent coke. The vapour of bisulphide of carbon is very heavy—its 
specific gravity being about 2-6447; and its volatility is such that it cannot be condensed 
from gas by cold. It is not soluble in water, but it is freely dissolved by alcohol and 
ether, and by the volatile and fixed oils. When the vapour of sulphide of carbon is 
mixed with aqueous vapour, and passed through a red-hot tube, it is decomposed, and, 
by a mutual interchange of elements, the sulphur is converted into sulphuretted hydro¬ 
gen, and the carbon into carbonic acid. It is also decomposed when it is passed over 
red-hot lime, or baryta, and when it is brought into contact with the peroxides of the 
metals—as of iron, manganese, and tin, in an ignited state ; and, lastly, I may say that 
it is absorbed by sulphide of ammonium, and that it is decomposed by an alcoholic or 
aqueous solution of the alkalies in the presence of a salt of lead. All these reactions 
have been made the basis of inventions for the removal of bisulphide of carbon from 
coal gas. Thus the solution of it in oil is the property taken advantage of it in the pro¬ 
cess of Schomberg, who proposes that the gas should be washed with oil. The decom¬ 
position of it at a red heat by steam is the recent suggestion of Mr. Lewis Thompson. 
The decomposition by ignited lime is the process of Mr. Bowditch ; and its solution in 
ammoniacal liquor is the plan proposed by your president. The absorption of it by an 
alcoholic solution of potash or soda is the patent of Dr. Stenliouse; and the decomposi¬ 
tion of it by passing it through a solution of oxide of lead in soda is the process of Dr. 
Angus Smith. And, although none of these processes have been found to be absolutely 
effective in practice, yet they are all more or less so; and they point to the importance 
of removing, as far as possible, this objectionable impurity from coal gas : for, in the act 
of burning, the sulphur of the bisulphide is converted into sulphurous acid, and this by 
further oxidation, in the presence of moisture, soon becomes sulphuric. 
The tests for the discovery of this compound in coal gas are very numerous, and are 
founded on the reactions before mentioned. Dr. E. Herzog, for example, recommends 
that the gas should be passed through a strong solution of ammonia in alcohol, to which 
a few drops of sugar of lead have been added; and, if the bisulphide be present, it will 
give an orange-red precipitate. Or the gas may be passed into an alcoholic solution of 
potash, which freely absorbs the bisulphide, and converts it into xanthate of potash, 
which produces, with a solution of copper, the characteristic yellow precipitate of xan¬ 
thate of copper ; and, when boiled with a little sugar of lead, it produces a brown or 
black precipitate. Dr. Hofmann’s test for bisulphide is to pass the gas through an 
ethereal solution of triethylphosphine, which forms with it a compound that crystallizes 
VOL. vii. z 
