468 
HOLGATE: SOME PHYSICAL PROPERTIES OF COAL, 
They are more suitable for lires working at a low temperature, 
and require frequent stirring. They contain more fusible salts that 
have been deposited by the passage of water between the numerous 
interstices than the dull coals do, and owing to the presence of these 
salts the ash clinkers in combustion. They intumesce and cake, 
thus preventing a free and plentiful passage of air. They make 
much smoke. 
Coals which are very soft and break easily into small, make the 
hardest and most valuable coke. If the cleavage is in more than one 
direction, causing the coals to break down into dust, they make the 
best coke. Hard coke may be made artificially from coal that would 
naturally only produce one of a spongy character, by making it into 
dust, and washing it with water before burning it into coke. 
The cleavage has been produced by lateral pressure taking place 
in difterent directions, and at various times during the long period 
that has elapsed since the strata were deposited, which has caused 
repeated elevation and depression. Coals obtained in a district which 
is much faulted or folded, ,and in the neighbourhood of volcanic 
action, break into small, and give off less gas, but make better coke 
than do those found in a district where the geological disturbance 
has not been great ; but the alteration is local. 
The relative position of the different kinds of coal in the same 
seam, whether top, middle, or bottom, has nothing to do with the 
({uality ; this depends entirely upon the original structure of the 
coal and the changes through which it has since passed. For the 
same reason the position of the coal, whether at a gTeat depth or 
near the surface (unless so near that air and water have affected it), 
does not influence its quality. Thin coal, and coal of inferior quality, 
though found, is not worked at great depths, because it would not pay 
to do so ; the pit therefore is sunk through it and it is left. It does 
pay to work a thin or inferior coal near the surface if it is in such a 
position that a ready sale may be obtained on or near the spot. 
