72 Professor Harkness on Coal. 
substance, to a very great extent, justifies, more particularly when 
we come to consider the structure which this presents. 
The Associated Mineral Matter—Having seen the natures 
of the vegetable structures which enter into the composition of 
coal, and how this may be variously modified by chemical changes 
and compression, we are next led to enquire into the results 
which are produced on the compressed and chemically altered 
vegetable matter by the addition of earthy substances. These 
earthy substances make their appearance as ashes, or in some modi- 
fied form of solid products, which remains after combustion; and as 
the amount of ash varies greatly in different coals, so does the amount 
of earthy matter which in part forms this substance. The numerous 
analyses of the various coals which have been ascertained, would 
occupy too large a space, and therefore a few of these may be given, — 
showing how greatly the quantity of earthy matters varies. With 
.regard to cannels, those which occur in the Scotch coalfield afford 
the following results :—Rochsoles 17 to 32 per cent., Methill 18 to 
223 per cent., Boghead 20 to 25 per cent., Dryden 253 per cent., 
Balbardie 26 per cent., Hillhead 27 per cent., Capeldrea 20 to 30 
per cent. In the case of the English cannel coals, these also vary 
greatly in the amount of earthy matter which is contained in them. 
One of these, the Hoo cannel of Wigan has, in the Botton Hoo, 22 
per cent., and in the top Hoo as much as 40 per cent. The com- 
mon coals also vary greatly in the amount of earthy substances which 
enter into their composition, the Scotch varying from 10°70, as in 
the case of the Wallsend Elgin, to 3°10 in the Dalkeith coronation 
seam. The same difference prevails among the English ordinary 
coals, and also in those which are obtained from the South Wales 
coalfield. However, as respects ordinary coals, these afford a smaller 
amount of ash than is procured from the cannels, Many coals which 
are in no way possessed of the characters of cannel coal, in some 
instances furnish a very large quantity of ash, approaching to 380 
per cent. ; and when this prevails to such an extent, it materjally 
modifies the character and value of coal. The compound nature of 
coal prevents us regarding this as a mineral, in the strict sense of 
the term, however the popular idea may be expressed on this sub- 
ject ; and being of a compound nature this substance passes from coal 
to shale in proportion as the mineral matter with which the organic 
constituents of coal is associated predominates ; and when this mine- 
ral matter prevails to such an extent as to render this compound of 
chemically altered and compressed vegetable matter, associated with 
earthy substances, incapable of being used as fuel by itself, it should 
be regarded as a coaly shale ; a substance which has, unfortunately — 
for science, been termed bituminous shale, Between coals and 
shales, therefore, there is an insensible gradation, and the point 
where the one ends and the other begins can only be determined 
by ascertaining whether this compound substance can be used for 
the ordinary purposes of fuel. 
