it, in fully one-half, might I not say nine-tenths, of existing furnaces and close stoves.  But in an ordinary gas retort the heat required to distill the gas is furnished by an outside fire; this is only necessary when you require lighting gas, with no admixture of carbonic acid and as little carbonic oxide as possible.  If you wish for heating gas, you need no outside fire; a small fire at the bottom of a mass of coal will serve to distill it, and you will have most of the carbon also converted into gas.  Here, for instance, is Siemens’ gas producer.  The mass of coal is burning at the bottom, with a very limited supply of air.  The carbonic acid formed rises over the glowing coke, and takes up another atom of carbon to form the combustible gas carbonic oxide.  This and the hot nitrogen passing over and through the coal above distill away its volatile constituents, and the whole mass of gas leaves by the exit pipe.  Some art is needed in adjusting the path of the gases distilled from the fresh coal with reference to the hot mass below.  If they pass too readily, and at too low a temperature, to the exit pipe, this is apt to get choked with tar and dense hydrocarbons.  If it is carried down near or through the hot fuel below, the hydrocarbons are decomposed over much, and the quality of the gas becomes poor.  Moreover, it is not possible to make the gases pass freely through a mass of hot coke; it is apt to get clogged.  The best plan is to make the hydrocarbon gas pass over and near a red-hot surface, so as to have its 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.