SECTIONAL TRANSACTIONS.—C. 371 
Dr. J. G. Kine. 
Whatever success may attend microscopic investigations employing the 
latest advances in photographic technique, it must be realised that they give 
information only on the physical structure of coal, whereas it is the chemical 
constitution of the several ingredients which finally determines the nature 
of the coal. The lack of methods for the complete separation of these 
various constituents is, however, a formidable obstacle. 
Constancy of elementary composition of any constituent is not a criterion 
of constant constitution. 
Professor Hickling has discussed the significance of the variation of 
rank across a coalfield, and defines ‘ rank’ as the degree of alteration which 
has occurred in the original plant débris during its transformation into coal. 
There is no direct measure of rank ; but in general the youngest (brown) 
coals are the richest in oxygen, and there is a steady diminution in oxygen 
as the coal ‘ matures.’ The oxygen content of coal can be varied, as, for 
instance, by weathering. Thus the plotting of volatile matter, etc., does 
not provide a complete means of tracing variations in rank across a coalfield. 
A ‘molecular’ weight would be valuable in measuring rank; but until 
colloid chemists provide the necessary methods, it would be better to avoid 
using the term. 
Mr. L. SLaTer. 
There is an important relationship between the rank of coal and its micro- 
scopic characteristics. In the Coal Research Laboratory at Sheffield 
thin sections have been made of a Leicester coal of 79:5 per cent. carbon 
content, many Yorkshire coals with a range of 81 to 88 per cent., Kent coals 
of 86:7 to 91°4 per cent., and South Wales coals of 87 to 93:5 per cent. 
There is a steady decrease in translucency as rank increases and it becomes 
increasingly difficult to differentiate between the types of plant constituents 
of which the coal was originally composed. In the lowest rank members 
of the series practically every microscopic constituent can be rendered 
translucent with comparative ease, and the brilliant yellow of the spores 
isin sharp contrast with the pale red of the ‘ vitrinite.” With increasing rank 
the translucency of the whole diminishes and the colour contrasts become 
less marked : when a rank of about 91°5 per cent. of carbon is reached the 
spores have practically the same colour as the ‘ vitrinite’ and are distin- 
guishable only by their outline against an opaque background. Thin 
sections of coals of higher rank than this are of little value as organised 
structure is practically invisible, and etching must be resorted to. Rank 
then determines the degree to which etching must be carried out in order 
to reveal structure clearly. Thus a coal of 94 per cent. carbon must be 
subjected to a given etching fluid for three times as long as a coal of 91 per 
cent. 
The diminution in volume undergone by a coal in increasing in rank 
from '79°5 to 93°5 in carbon is remarkably slight, probably less than 10 per 
cent. 
Dr. R. LEssINnc. 
The study of the development of rank in coal is the chemist’s domain, 
in which close co-operation with the palzo-botanist and geologist is essential. 
The principal reactions involved in raising the rank of coal are the elimination 
of water and CO,, and later the loss of other volatiles under the influence 
of the particular geological environment obtaining. The speaker has shown 
