The nature of the fuel required depends on the purpose for which the gas is used. If for heating boilers, furnaces, etc, coke or any kind of coal maybe used; but for gas engines or any application of the gas requiring great cleanliness and freedom from sulphur and ammonia it is best to use anthracite, as this does not yield condensable vapors, and is very free from impurities. Good qualities of this fuel contain over 90 per cent of carbon and so little sulphur that, for some purposes, purification is not necessary. For gas engines, etc., it is, however, better to pass the gas through some hydrated oxide of iron to remove the sulphureted hydrogen. The oxide can be used over and over again after exposure to the air, and the purifying is thus effected without smell or appreciable expense. Gas made by this process and with anthracite coal has no tar and no ammonia, and the small percentage of carbon dioxide present does not sensibly affect the heating power. A further advantage of this gas is that it cannot burn with a smoky flame, and there is no deposition of soot even when the object to be heated is placed over or in the flame, and this is of importance for the cylinder and valves of a gas engine. To produce 1,000 cubic feet only 12 lb. of anthracite are required, allowing 8 to 10 per cent, for impurities and waste; thus a generator A size, which produces 1,000 cubic feet per hour, needs only 12 lb. in that time, and this can be added once an hour or at longer intervals. No skilled labor is necessary, and in practice it is usual to employ a man who has other work to attend to near the generator, and to pay him a small addition to his usual wages. The comparative explosive force of coal gas and the Dowson gas calculated in the usual way is as 3.4:1, i. e., coal gas has 3.4 times more energy than the writer's gas. Messrs. Crossley, of Manchester, the makers of the Otto gas engines, have made several careful trials of this gas with some of their 3½ horse power (nominal) engines, and in one trial they took diagrams every half-hour for nine consecutive days. These practical trials have shown that without altering the cylinder of the engine it is possible to admit enough of the Dowson gas to give the same power as with ordinary coal gas. It has been seen that the comparative explosive force of the two gases is as 3.4:1, but as it is well known the combustion of carbon monoxide proceeds at a comparatively slow rate, and for this reason, and because of the diluents present in the cylinder which affect the weaker gas more than coal gas, experience has shown that it is best to allow five volumes of the Dowson gas for one volume of coal gas, and then the same uniform power is obtained as with the latter.