370 SECTIONAL TRANSACTIONS.—C. 
no relation to the thickness of Permian and Triassic strata lying uncon- 
formably above them. 
Thermal, dynamic, and pneumatolytic metamorphic changes, as recog- 
nised by those who work on crystalline rocks and mountain building, are 
manifestations of variations of stress and temperature conditions through a 
wider and higher range than those responsible for advance of rank in English 
coals. It is possible that as water was expressed from consolidating sedi- 
ments, loosely combined water and carbon dioxide may have been removed 
from organic constituents of vegetable debris, but chemical changes have 
not been noted in the mineral constituents of associated rocks. 
Prof. W. A. Bong, F.R.S. 
Recent researches in my laboratories at the Imperial College of Science 
and Technology, London, besides proving the benzenoid structure of the 
main coal substance, have shown that such structure, which originated in 
lignin (but not in cellulose), has been progressively developed throughout 
the series lignin-+peat—brown coal-+lignite>bituminous coal—anthracite, 
and thus supports the continuity of the series. Also they have shown that 
the constituents mainly responsible for the coking propensities of bituminous 
coals have probably originated in, and been developed from, the phenolic 
constituents of brown coals. Our researches have disclosed no essential 
or material difference between the chemical constitutions of the ‘bright’ 
and ‘ dull ’ components, i.e. the ‘ vitrain ’ and ‘ durain,’ of bituminous coals ; 
and however apt or convenient such names may be from the morphological 
standpoint, it is doubtful whether they have any particular chemical 
significance. 
The chemical nature and constitution of coal are questions quite apart 
from any morphological nomenclature yet suggested, and can be solved 
only by the persistent application of the experimental methods of organic 
chemistry ; they may ultimately prove to be but little related to the petrology 
of coal. 
Probably ‘ pressure ’ has been a more potent factor than ‘ temperature ’” 
in the maturing of the original coal substance after the ‘ peat-bog stage.’ 
It is hoped to put this matter to an experimental test shortly, as means and 
apparatus are now available for studying organic reactions under pressures 
between 10,000 and 20,000 atmospheres. 
Mr. F. V. TIDESWELL and Prof. R. V. WHEELER. 
The ‘rank’ of a coal is a conception expressing the general level of a group 
of its properties and in its broadest sense is a measure of the extent of chemical 
alteration of the coal-forming ingredients from their original composition. 
This progressive chemical alteration (i.e. change of rank) is a property of 
the ulmins which form the bulk of all coals, and the changing properties of 
coals of different ranks can be attributed almost entirely to this progressive 
change in the ulmin ingredient. Rank can therefore be assessed by 
measuring properties peculiar to the coal ulmins, for example, oxidisability, 
or decomposition temperature, although the chemical changes responsible 
for the alteration in rank of the ulmins are as yet not fully known. 
The decomposition temperatures of coal ulmins from secondary or more 
recent deposits in New Zealand agree well with those of our British Palzo- 
zoic coals of corresponding composition, confirming the similarity of the 
processes involved in alteration of rank despite wide differences in geo- 
graphical position and geological period. 
