holgate: some physical properties of coal. 
461 
purposes in wliicli smoke is undesirable, or in whicli a strong draught 
or blast of air is introduced for the heating of substances in the body 
of the fire. 
Another way in which coals are used is in the Sienien's furnace ; 
in this case the coal is burnt in an ordinary fire-grate, but with such a 
small quantity of air that the gas given off is hydrogen aud carbon- 
monoxide. The completion of the combustion, that is, the changing 
of the carbon from the monoxide into the dioxide or carbonic acid, 
and the hydrogen into water, being completed in the l)ody of the 
furnace, in the presence of the iron, steel, glass, or other substance 
which is to be melted. 
Turning from this to the different kinds of coal, we find them 
equally varied : lirst we have the cannel coals which are found in 
n(ui-continuous basins or hollows, it may be along with other coals 
or in seams by themselves. Black in colour, they vary in appear- 
ance from being shiny, light in weight, and weak, breaking with 
a conchoidal fracture, to being d\\\] in ai)pearance, black, and dense, 
but whatever the difference in their appearance they are alike in 
containing remains of fishes or the shells of molluscs, and it is to the 
fact of their containing animal hydro-carbons tliat in combustion 
they give a white light, and for that reason are used specially for gas 
making owing to the high illuminating power of the gas made from 
them. The temperature at which the dull-looking cannel gives off 
its gas is much lower than that at wdiich its solid part fuses, and the 
consequence is that the coke is not changed in form when it comes 
out of the retort from that at which it was introduced into it ; besides 
this, as the coal contains a certain amount of mud, the ash is large in 
quantity and white in colour. 
The cannels then were formed under w^ater, and perhaps of 
masses of floated wood, combined with the remains of fishes, but 
mostly of trees and water plants that grew in situ but in a sinking 
area and under water in which fishes and molluscs had their habita- 
tion, and into which a small portion of fine mud found its way; since 
that time the animal hydro-carbons have permeated the vegetable 
matter and the mud, and a hard, compact coal has been formed. There 
are several seams of coal of this kind in the vicinity of Leeds. 
