466 IIOLGATE : SOME PHYSICAL PKOPEHTIES OF COAL. 
made in the circumstance under which it is at present used ; if how- 
ever it were used for boiler firing, its gases would be brought during 
combustion into contact with comparatively cold boiler plates, and we 
may safely say that it would then make a considerable amount of smoke. 
When these furnaces were first introduced it was claimed for 
them that small coals would be as well adapted as large, but this was 
soon found not to be the case. Small coal is in no case as good as 
large, when the object is to obtain a large quantity of volatile gas, 
either for combustion or gas" making ; but small coals make the best 
coke. 
The question wdiich at once presents itself is, How does this 
cleavage take place, and^why is it different even in the same seam 
of coals ? 
I will answer the last question first. The whole of the coal 
measures sliow^ us upon examination that they were in a perpetual state 
of change during deposition, the land, low lying as it was, and subject 
not only to floods but to changes in the courses of its larger rivers, 
changed its vegatation to suit the changing conditions, and coals are 
made up of a great variety of plants, sometimes of huge trees, 
sometimes of water-plants with their attendant fish remains ; and 
though all went to form coal, they would naturally make different kinds. 
When we find beds of fish and shell-remains in the shale w^e see 
that the oil from these animals has permeated through the shale 
hardening it into a marble, and it has had the same effect on the coal 
in its immediate vicinity. 
If we examine coal in the neighbourhood of a fault, we find that 
it has been cloven in more than one direction, and often it has been so 
compressed that it has been shattered. In other places we may see lines 
of cleavage in two totally different directions, and this not only affects 
the coals but all the strata of shales, sandstones, etc. We know that 
the land has been repeatedly raised and lowered, and that the pressure 
causing this has been a lateral one, and we know that wherever we 
find contortion of the strata, or a proximity to volcanic action, the 
coals are changed as regards their cleavage. 
If we examine a piece of Welsh anthracite we do not wonder that 
it will not blaze, for though bright in appearance, we see that it has 
