heaviest hydrocarbons decomposed, but so as to leave all those which are
able to pass away as gas uninjured, for it is to the presence of these that
the gas will owe its richness as a combustible material, especially when
radiant heat is made use of.

The only inert and useless gas in an arrangement like this is the nitrogen
of the air, which being in large quantities does act as a serious diluent.
To diminish the proportion of nitrogen, steam is often injected as well as
air. The glowing coke can decompose the steam, forming carbonic oxide and
hydrogen, both combustible. But of course no extra energy can be gained by
the use of steam in this way; all the energy must come from the coke, the
steam being already a perfectly burned product; the use of steam is merely
to serve as a vehicle for converting the carbon into a convenient gaseous
equivalent. Moreover, steam injected into coke cannot keep up the
combustion; it would soon put the fire out unless air is introduced too.
Some air is necessary to keep up the combustion, and therefore some
nitrogen is unavoidable. But some steam is advisable in every gas producer,
unless pure oxygen could be used instead of air; or unless some substance
like quicklime, which holds its oxygen with less vigor than carbon does,
were mixed with the coke and used to maintain the heat necessary for
distillation. A well known gas producer for small scale use is Dowson's.
Steam is superheated in a coil of pipe, and blown through glowing
anthracite along with air. The gas which comes off consists of 20 per cent.
hydrogen, 30 per cent. carbonic oxide, 3 per cent. carbonic acid, and 47
per cent. nitrogen. It is a weak gas, but it serves for gas engines, and is
used, I believe, by Thompson, of Leeds, for firing glass and pottery in a
gas kiln. It is said to cost 4d. per 1,000 ft., and to be half as good as
coal gas.

For furnace work, where gas is needed in large quantities, it must be made