LIQUID, SOLID, AND GASEOUS FUELS FOR POWER PRODUCTION. 809 
required into the pre-heated air, and the oil is readily freed from foreign 
matter; also there does not appear to be any inherent reason why the 
oil-engine should not be made in the largest sizes beyond those of 
weight and cost, where it must always be inferior to the rotary 
machine, 
There remains now to consider the question of the gas-turbine. It 
can be readily shown that the velocities which have to be dealt with 
are no greater than those encountered in the steam-engine, and where 
then come the difficulties which, in spite of innumerable efforts, have 
so far proved insurmountable. 
The greatest defect is that so far it has not been found feasible to 
compress the air and gas in the turbine itself. Separate compression 
considerably increases the losses, so that in place of the negative work 
being one-third of the gross work, it would probably be at least one-half, 
and perhaps more. The cooling of the rotating disc and blades offers 
difficulties, and also so far no material has been found that will 
withstand the erosive effect of the burning gases. It seems very 
doubtful if the gas-turbine can be constructed to compete with the 
reciprocator on the present state of knowledge. 
It is important to consider the amount of various fuels which are 
raised in various parts of the world. At present about twelve hundred 
million tons of coal of various kinds are brought to the surface every 
year. Crude oil amounts to about fifty million tons per year, and it 
is doubtful if there are any large oilfields yet to discover; it therefore 
follows that the supply of oil is totally inadequate in amount to replace 
coal for power production on a scale equal to the present steam-power 
production. 
The artificial production of oil is one that at present is being con- 
sidered seriously by engineers, and there is very little doubt that much 
yet remains to be discovered in this field. Every engineer is familiar 
with the fact that when coal is heated in a closed retort gas and tar 
are given off, and also that the higher the temperature at which 
gasification takes place the greater is the yield of gas and less tar 
is formed. Tar when distilled gives a number of valuable substances, 
but only a small fraction of light oils; the middle oils contain some 
cresylic acid which is suitable for the Diesel motor. 
Up to the present coal has been gasified with the object of making 
the highest yield of gas, but it is quite possible to alter the conditions 
of carbonisation so as to obtain a high yield of fuel oils and other 
compounds of value. As a side issue, but an important one, is the 
production of sulphate of ammonia, the use of which as a manure is 
steadily increasing in all countries of the world. 
Sulphate of ammonia is the most valuable manure where nitrogen 
is needed, and the author considers it is one of the greatest defects 
of the crude burning of coal that such enormous amounts of nitrogen 
are not only wasted, but turned into nitric acid in the atmosphere. 
Of course the nitric acid is returned to the soil by the agency of rain, 
but not necessarily where required by the cultivator. 
Tt will now be necessary to consider the methods by which fuels 
may be utilised for the production of power. Oil has been shown to 
