330 
PRESENT STATE OF THE CHEMISTRY OF GAS-LIGHTING. 
in the form of little prisms of a splendid red colour. But the easiest mode of discover¬ 
ing its presence in coal gas, and also of determining its amount, is to burn the gas at the 
rate of 1 foot an hour from a Leslie burner, and to carry ..the products of combustion, 
together with ammonia, into a glass cylinder, where they are condensed, and where the 
sulphur is afterwards found as sulphate of ammonia. The proportion of sulphur thus 
discoverable in coal gas ranges from about 4 grains per 100 cubic feet in the better kinds 
of cannel gas, to about 40 grains in the worse varieties of common London gas. 
' (f) Sulpho-hydrocarbons. —The nature of these compounds has not been deter¬ 
mined, but there is good reason for concluding that sulphur exists in poal gas in other 
forms than as sulphuretted hydrogen and bisulphide of carbon. As in the last case, 
however, the sulphur, whatever may be its form of combination, is discoverable by the 
combustion process. 
(g) Sulphuretted Hydro gey (HS) is but rarely present in coal gas, and its propor¬ 
tion is never large. The test which is commonly used for its detection—namely, a little 
sugar of lead on paper—will discover the millionth part of this impurity. 
"(h) Cyanogen (C.,N).—This compound is found in but very small proportion in coal 
gas, although its presence is always indicated by the existence of sulphocyanogen in 
the water of the meters. It is a constant product of the carbonization of coals, and its 
proportion increases with the amount of nitrogen in the coal, and with the temperature 
at which they are distilled. Nearly the whole of the cyanogen is condensed in the hy¬ 
draulic main, where it unites with sulphuretted hydrogen and ammonia, and forms 
sulphocyanide of ammonium—the compound which gives the rich blood-red colour to 
the persalts of iron. But a trace of the gas escapes condensation, and passes into the 
mains, w'here it forms, in like manner, the sulphocyanide. In its pure state, cyanogen 
is a colourless gas, with a very peculiar odour, and it is a deadly poison. It consists of a 
volume of nitrogen and a volume of carbon vapour condensed into one volume, and its 
specific gravity is 1-8006. It bums with a pale rose-coloured flame, and forms twice its 
own volume of carbonic acid. It is not easily absorbed by water, but it is more- freely 
taken up by alcohol, ether, and volatile oils; and it is readily absorbed by alkalies and 
alkaline sulphides. It is this compound which, uniting with iron, forms Prussian blue, 
and so gives the blue and bluish-green colour to the spent lime (blue billy). There is 
no ready test for the recognition of the very small proportion of cyanogen contained in 
coal gas ; but if a large volume of gas be passed through a tube containing flints mois¬ 
tened with an alkali, and the flints be acidified, they will acquire a blue colour if iron be 
present, in consequence of the formation of Prussian blue ; or the liquid may be treated 
with a little sulphate of iron, and then acidified with sulphuric acid, when it will present 
a blue colour. 
(i) The last impurity in coal gas is Aqueous Vapour (HO), the proportion of which 
varies with the temperature. It is rarely less than 200 grains, or about 1037 cubic 
inches, in 100 cubic feet of gas, and it rarely exceeds 600 grains, or 3111 cubic inches, 
in the 100 feet. The influence of this impurity is not very striking, although the ten¬ 
dency of - it is to reduce the luminosity of the flame by decomposing the hydrocarbons in 
the burning gas. 
There is one property of aqueous vapour which has not been sufficiently noticed, but 
which is the chief cause of the heat and discomfort so often experienced in a room where 
gas is largely consumed. The property which I allude to is that of absorbing the radiant 
heat from the burning gas, and so, by its distribution through the air, elevating the 
temperature of the room. When much aqueous vapour exists in the atmosphere—and, 
as you will see by the table already referred to, there is much of it produced by burning 
gas—there is not only an elevation of temperature by the absorption of the radiant heat 
from the gas-flame, but there is also a check to the natural evaporation from the surface 
of our bodies, and the perspiration collects as visible moisture. We, therefore, feel hot 
and uncomfortable. This is another reason why the products of the combustion of gas 
should be removed as speedily as possible from the room in which they are produced. 
And now I have but little time left for the discussion of the second part of my subject 
—namely, the means to be used for the combustion of gas so as to secure the fullest 
amount of heat or light. I will, however, direct your attention to two or three experi¬ 
ments, which will illustrate the fact that the light of a flame is dependent on the num¬ 
ber of minute solid particles within it, and on the intensity and duration of their ignition. 
On the one hand, if I take hydrogen gas, which contains no solid particles, and which 
