holgate: some physical phoperties of coal. 467 
been so compressed and is so slicken-sided in all directions that we 
can plainly judge the cause from the result. 
Belgian coals, which are worked almost vertically owing to 
contortion, are also much shattered. 
The coals which are made into coke by Messrs. Strakers & Love, 
in West Durham, have been so shattered that they fall into dust if 
exposed to the weather, and they may be easily crushed in the hand, 
but though they are very poor as coal, they make the best and hardest 
coke in the country. 
In order to make coke from the coals of our own and the Shef- 
field district which naturally cleave at greater distances apart, it is 
absolutely necessary that they should be small, the smaller the better, 
as by this means the gasses are not allowed to escape, but are con- 
densed into the coke. It is also necessary that they should be w^ashed, 
not only so as to wash away any dirt, but also any chemical impurities 
and some volatile gasses whose presence would result in an inferior 
coke. A very good coke suitable for melting steel and iron is made 
in this w^ay. 
The cleavage spoken of above must have taken place a long time 
ago, for we find that water containing lime, barium, and sulphide 
of iron has percolated along the interstices of the cleavage, and has 
left these substances in a crystalline form. This is another source of 
inferiority of* coals having their planes of cleavage close together, 
they not only break into small, but they often contain a large quantity 
of deposited impurities 
From what has been said we may draw the following conclusions : 
the mode of burning and consequent usefulness of coal is governed 
principally by the sizes into which it breaks down before or after 
being thrown into the furnace. 
Coals dull in appearance having a brown streak and containing 
many spores and fish remains are tough and break into large pieces. 
They are suitable for furnaces working at a high temperature, and 
allow the air to pass freely through them in the furnace. They are 
free burning. 
Bright coals, having cleavage planes close together, are soft and 
break into small, either before or after they are thrown into the furnace. 
