58 SECTIONAL ADDRESSES 
ever seriously contended that either set of circumstances has not been 
responsible for some share in producing differences in quality ; but there 
has been the most extreme divergence of opinion as regards the relative 
importance to be assigned to each. From the geological standpoint the 
greatest contribution which is made by the microscope to the study of coal 
is, that it enables us to determine definitely whether the difference in quality 
of two coals can or cannot be attributed to original difference of com- 
position. If they can be seen to consist of entirely similar plant materials 
in a similar state of preservation, then any considerable difference in 
quality must be attributed to some other factor. Conversely, we may 
compare the quality of coals which are visibly composed of different 
materials, and find what are the constant differences between them. 
Composition of the several Components.—In common with most vege- 
table substances, coals consist mainly of carbon, hydrogen and oxygen, 
and as the first step in the discussion of this problem we may enquire 
how the proportions of these constituents vary among the different plant 
materials which may be isolated from any single piece of coal. All the 
vitrinite is found to be of very uniform composition. The group of 
cuticles, spores and resins is notably high in hydrogen, but is not ex- 
ceptional in the ratio of carbon and oxygen. Any fusain present will be 
very deficient in both hydrogen and oxygen. In a coal in which the 
general percentage of oxygen is 15, that of the fusain may be as low as 5. 
The chemical composition of the residuum needs further investigation ; 
the indications are that it is usually a little deficient in hydrogen and 
oxygen, but not to such an extent as in the case of fusain. The outstand- 
ing fact is that the different components vary strikingly in their hydrogen 
content (from about 3-5 per cent. in fusain to 8-o per cent. in some spores), 
while only in the case of fusain is there any notable variation in the oxygen. 
Composition of the Aggregate-—How does this variation in the composi- 
tion of the individual plant materials compare with the observed range of 
the bulk composition of different coals? If we take a very large number 
of representative analyses of lignites and bituminous coals (but exclude 
for the moment anthracites and cannels) we shall find the limits of varia- 
tion of the chief constituents to be approximately as follows : Hydrogen, 
4°5 to 6-0 per cent.; carbon, 65 to go per cent.; oxygen (varying in 
almost exactly complementary proportion to the carbon) 30 to 5 per cent. 
We see clearly that the whole variation of hydrogen content is well within 
the range observed among the different plant constituents in a single 
piece of coal. There is consequently no reason to suppose that among 
this group of coals the hydrogen content has been appreciably varied 
except by the character of the original materials. Variation in the type 
of coal shows itself in the chemical analysis chiefly in differences of hydro- 
gen content, and it might account for the whole of those differences. 
But the answer is emphatically reversed when we turn to the 
carbon : oxygen proportions. A great increase in the proportion of fusain 
in the seam is the only change among the plant materials which could affect 
the general oxygen content of the coal very considerably ; but so much as 
10 per cent. of this material would be an exceptionally high quantity in 
any seam, and this amount might reduce the general oxygen content of 
