IIULGATE: SOME PHYSICAL PROPERTIES OF OOAL. 465 
distance apart, whilst the coal which presents a shiny, polished sur- 
face on its broken flice (not its plane of cleavage) is soft and brittle, 
and has its cleavage planes in close proximity. Between these two 
kinds we find every variety. 
In the Middleton Main and Haigh Moor, the coals are 
made up of alternate thin layers of the two kinds, the result being, 
a coal not so tough and strong as if made up principally of 
spores, and yet much stronger than are the bright coals above- 
named ; it is to this difference in the distance between the planes of 
cleavage that the usefulness of the coal depends; those which have 
their cleavage planes wide apart break naturally into larger pieces as 
above described, and give very different results when used, either for 
combustion or for gas-making, from those which cleave naturally into 
thin plates. Coal of the latter kind if thrown on a fire even in large 
pieces, by breaking up at once into small when warmed, and by pre- 
venting a free passage of air, gives the inferior result of a short 
flame at a low temperature, with a good deal of black smoke. Under 
these circumstances we are led to the conclusion that coals having 
pretty nearly the same ultimate chemical constitution, but differing in 
the distances between the planes of cleavage, give totally different 
results during combustion. Tlie coals which thus naturally break 
into small, if used for gas-making, will deliver their gas more slowly, 
and this gas, as it is liberated, instead of clearing itself and working- 
its way between the pieces of coal, which it would do if they were 
large, makes different chemical combinations through its being 
smothered by the surrounding small coal, and so we get less gas. 
With coals of this kind, notwithstanding the greatest care in firing, 
much smoke will be made, and every means that has as yet been 
tried to burn the smoke in furnaces of the ordinary kind, when 
using such coals have resulted not in economy but in loss of heat 
and consequently of work. 
The reason why Leeds looks so black is accounted for by the 
fact of so many coals being used here that naturally break into small, 
as we have described, and so are smoky. 
I do not here refer to the Siemen's furnace, in which a far more 
■perfect distillation takes place, and in which but little smoke is 
K 
