86 
PO POLAR SCIENCE REVIEW. 
Coal is by no means the only, though it is certainly the 
principal, material from which gas is obtained. Bituminous 
shales, oil, resin, peat, and wood, are all capable of yielding a 
certain supply ; and some of these substances, badly adapted for 
fuel, are extremely valuable for illuminating purposes, owing to 
the large quantity of light carburetted hydrogen gas that may 
be obtained from them. The presence of this gas in the actual 
pores of coal, whence it is given off in large quantities, is often 
intimated underground by a peculiar singing noise, and in some 
mines a naked light applied, to freshly-cut coal will actually 
produce a flame from numerous small jets. This is probably 
owing to the great pressure brought to bear upon the remain- 
der, when part of the coal is removed. A very much larger 
quantity of the same gas is obtained afterwards, by exposing 
the coal to intense heat in a retort, arranged so that the pro- 
ducts of distillation shall be received in convenient vessels for 
the purification of the gas, and afterwards transmitting it by 
pipes to the place where it is required for burning. 
Although, however, the process of obtaining gas that can be 
rendered useful for illmnination is so simple, that every school- 
boy has made the experiment in the bowl of a tobacco-pipe, 
the mechanical difficulties of applying it on a large scale were 
at first exceedingly great, and have only lately been overcome 
in a satisfactory way. All the gaseous substances that are 
obtained from the combustion of the coal are by no means fit for 
burning, as they include, besides the gas we use in our streets 
and houses, several other gases, more or less noxious and use- 
less, and many vapours which require to be separated. Besides 
these, there are fluid, semi-fluid, and sohd products either carried 
over or left behind. Even the illuminating gases themselves 
are many in number, and vary in them properties, some having 
a disagreeable odour, some being unwholesome and therefore 
objectionable for general use, and others exceedingly valuable 
as giving pure white light without adding to the heat of the 
mixture during combustion. The essential ingredients of illu- 
minating gas are carbon and hydrogen ; but all true coal con- 
tains, besides these, both oxygen and nitrogen gas and sulphur. 
These elements, either alone or hi various new combinations, 
are obtained after rapid distillation at high temperature, so that 
watery vapour, ammonia, sulphuretted hydrogen, carbonate of 
ammonia, and a variety of compounds, of which paraffine and 
benzole are the best known, come off with the illuminating gas, 
and may be collected. They are present in quantities that vary 
according to the nature of the coal, the temperature employed 
in distilling’, and the length of time occupied in the manufac- 
ture. 
Not only, therefore, is there left behind in the retort a 
