arches are put into the
producers a certain distance down, and the fuel is kept above the
bottom level of these hanging arches. This compels the products of
distillation, produced when fresh fuel is charged in, to pass through
the incandescent fuel between the two hanging arches, whereby the
tarry products are to a considerable extent converted into permanent
gas, and the coal dust arising from the charging is kept back in the
producer.

The details of construction of this plant will be easily understood by
reference to the diagrams before you.

The fuel we use is a common kind of slack, and contains, on an
average, 33.5 per cent. of volatile matter, including water, and 11.5
per cent. of ashes, leaving 55 per cent. of non-volatile carbon.

The cinders which we take out of the producer contain, on an average,
33 per cent. of carbon. Of this we recover about one-half by riddling
or picking, which we return to the producer. The amount of unburnt
carbon lost in the cinders is thus not more than 3 per cent. to 4 per
cent. on the weight of fuel used.

The gas we obtain contains, in a dry state, on an average, 15 per
cent. of carbonic acid, 10 per cent. of carbonic oxide, 23 per cent.
of hydrogen, 3 per cent. of hydrocarbons, and 49 per cent. of
nitrogen.

The caloric value of this gas is very nearly equal to 73 per cent. of
the caloric value of the fuel used, but in using this gas for heating
purposes, such as raising steam or making salt, we utilize the heat it
can give very much better than in burning fuel, as we can completely
burn it with almost the theoretical quantity of air, so that the
products of combustion resulting do not contain more than 1 to 2 per
cent. of free oxygen. Consequently the heat escaping into the chimney
is very much less than when fuel is burnt direct, and we arrive at
evaporating, by means of the gas, 85 per cent. of the water that we
would evaporate by burning the fuel direct, in ordinary fireplaces.

We have, however, to use a certain quantity of steam in the producers
and in evaporating the sulphate of ammonia liquors, which has to be
deducted from the steam that can be raised by the gas in order to get
at the quantity of available steam therefrom obtainable. The former
amounts, as already stated, to 0.6 ton, the latter to 0.1 ton of steam
per ton of fuel burnt, making a total of 0.7 ton. The gas obtained
from one ton of fuel evaporates 5.8 tons of water in good steam
boilers, working