52 SECTIONAL ADDRESSES 
names are most potent things, especially if they are apt. These names 
directed attention sharply to the facts that common coal is a complex 
mass of different materials, and that some of the chief components (vitrain 
and fusain) or characteristic mixtures of components (clarain and durain) 
can be picked out of the coal and subjected to separate examination. 
They focused that attention and have thereby stimulated a very large 
volume of work on the nature and constitution of these components ; and 
it is no adverse comment to say that this work has necessitated some 
modification of their original definition. 
We can now revert to the ‘ uniform brown substance.’ Dr. Stopes 
observed that the clarain bands were mainly composed of this trans- 
lucent material, finely interspersed with other ingredients, and often 
showing its original plant structure. Vitrain consisted entirely of similar 
translucent material, free from admixture with other substances, but 
seemed to show no trace of structure. 
The development of technique during the past decade has shown, 
however, that this latter distinction was due largely to the physical 
properties of the vitrain. The pure unmixed substance of these bands 
is excessively brittle and most difficult to section effectively ; but it is 
now abundantly shown that a very large proportion of the vitrain, at 
least, retains the structure of the tissue from which it has, in fact, been 
formed, no less clearly than the smaller fragments of translucent material 
which enter into the composition of the clarain bands. Ina recent classifi- 
cation of coal ingredients (January 1935), Dr. Stopes has indicated clearly 
that the chief part of the substance of the clarain bands appears to be 
identical with that which forms vitrain. In direct proportion as the 
methods of preparing thin sections or of etching coal surfaces have been 
improved, an ever-increasing proportion of the ‘ uniform brown sub- 
stance’ of the bright coal has been shown to retain the structure of the 
plants of which it is composed. While it is true that some small portion 
of the mass generally remains apparently structureless, the conviction 
becomes increasingly strong that this is a matter of appearance only. 
This conclusion is made more insistent by the observation that even when 
the structure is most faint and difficult to detect, this is due, not to any 
lack of perfection in the minutest details, but merely to want of contrast 
between the colour of the cell-walls and the material within the cells. 
The relation between vitrain and clarain, which has been the subject 
of very extensive discussion and much confusion, may be regarded as 
now cleared up. Subject to a qualification to be mentioned immediately, 
each separate band or lenticle of vitrain represents a single fragment of 
wood or bark or other piece of plant tissue. Its distinctness and its 
homogeneous character result directly from this fact. In other words, 
each plant fragment of sufficient size to be clearly visible on the coal 
surface constitutes a vitrain lenticle. Clarain, on the other hand, con- 
sists largely of similar plant fragments, which are individually too small 
to be distinguished without the microscope, and are moreover mixed 
with more minute plant debris such as spores, isolated cuticles and other 
matter. It is the minutely heterogeneous character of the mass which 
results in a lower surface lustre. 
