434 TRANSACTIONS OF SECTION B. 
unit with a capacity of fifteen tons per day was reached. The mechanical 
difficulties to be overcome as the scale of operations was increased were serious, 
and even in its present form we are not perfectly satisfied with the apparatus. 
We are now preparing designs for a further step in which we hope to profit 
by the experience of the past four years. We are, however, satisfied that the prin- 
ciple of exposing coal to heat in thin layers is sound. We are also satisfied that 
the production of a mechanically perfect apparatus into which small coal is auto- 
matically fed, passed through the distilling zone, and finally passed out through 
a cooling chamber, only requires a little more patient step-by-step development. 
It is obvious that an apparatus which could be built in units with a capacity of 
fifteen to twenty tons per day, which would work automatically, and no part of 
which need be exposed to a higher temperature than 450° to 500°, ought to pro- 
vide an exceedingly economical means for the distillation of coal. But I must 
not omit to tell you the weaknesses as well as the strength of this type of 
apparatus. It will, in its present form, only work smoothly with non-caking coal. 
If the coal, on heating, passes through a fusible stage it is apt to stick to the 
shelves and to accumulate on them. The working then becomes irregular, and 
eventually stops. Further, the fact that the coal is frequently turned over and 
dropped from shelf to shelf tends to break it down into small stuff, a good deal 
of which is no larger than coke breeze. These are both serious, but not fatal, 
disadvantages. 
The greater part of the coke from this unit plant has been used in water-gas 
producers into which it could be passed while it was still warm and dry. It had 
thus an initial advantage over the gas-works coke, which usually contains ten to 
fifteen per cent. of water. The use of the low-temperature coke for water gas- 
making proved quite satisfactory. Its light nature made it necessary to reduce 
the pressure of the air blast in the producer, but its freedom from water and its 
ready inflammability fully compensated for the loss of capacity due to this 
reduction in the air blast. 
A good deal of the low-temperature coke has also been converted into 
briquettes for domestic fuel. These are easily kindled and kept alight in an 
ordinary grate, and burn almost without smoke. The experience of numerous 
householders in Glasgow in the use of this fuel has been most encouraging, and 
my colleagues are quite satisfied that a steady outlet for a moderately large 
output could at once be obtained. 
The hydrocarbon gases from the unit apparatus have hitherto been passed 
into the general fuel gas system of the works, but regular laboratory tests have 
been made of the thermal value, petrol contents, &c. The thermal value of the 
gas reached the high figure of 850 B.T.U. per cubic foot. The liquid tar has 
been regularly collected and examined, and the results generally confirm those of 
other observers. 
Our attention has, however, been mainly concentrated on the mechanical 
development of this method of distillation, and on the production of a domestic 
or an industrial fuel from the coke. These, in my opinion, are the really funda- 
mental points in the low-temperature scheme. If these are not right then even 
fancy prices and an unlimited outlet for fuel oil and motor spirit will not save 
the scheme from failure. 
I must repeat that the really significant points are covered by the economic 
and engineering questions : can an outlet be found for the low-temperature coke? 
and can a satisfactory apparatus be devised ? 
The apparatus must be in fairly large units, and it must be automatic, and 
must work with the smoothness and regularity of the best types of automatic 
stoking machinery and with the minimum of manual labour or of detailed super- 
vision. The gases and vapours from the distillation must be carefully preserved 
from loss or damage in the apparatus either through leakage of gas outwards or 
of air inwards. The necessary heat must be so applied as to cause no deteriora- 
tion of the material of which the apparatus is constructed, and the heating must, 
of course, be economical. 
Dr. H. G. Cotman gave a general account of how far the gas industry was 
really helping towards the economic use of fuel. 
