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woods, therefore, are preferable to the softer for general 
purposes, for although the heat evolved by them is less 
than that given off by the softer kinds, it is rendered more 
useful in the furnace by passing off with less rapidity. 
But the greater intensity of the heat thus rapidly produced 
by tbe softer kinds, gives them a direct advantage in some 
manufactures, as in that of porcelain. 
The same remark applies equally to coals, and especially 
to those which are employed for naval purposes. It is often 
exceedingly important to be able to get up steam in a short 
time, and those coals which are admirably adapted for main- 
taining the fire when raised, are usually found deficient in 
this respect. A mixture of two coals of opposite properties 
is the only obvious remedy. 
I will now only detain you for a short time while I make a 
few remarks on the improvements in the development of heat, 
suggested by the preceding considerations. The destruc- 
tive distillation of coal, we have seen, is productive of the 
following compounds, — coke, tar, water, ammonia, carbonic 
acid, olefiant gas, hydro-carbons, and other inflammable 
gases. The following quantities are those resulting from 
analyses of six Welsh coals (one of which was anthracite), 
coke 85.99, tar 2.02, water 3.33, ammonia 0.18, carbonic 
acid 1.66, sulphuretted hydrogen 0.08, olefiant gas 0.24, 
other gases 5.34. Now we have seen that the heat de- 
veloped during the combustion of the gaseous constituents 
of a coal is equal exactly to the amount of heat abstracted 
during their formation. 
The existence of this thermo-neutrality corroborates the 
assertion, that the evaporative value of a bituminous coal is 
expressed by the evaporative value of its coke. This has 
been tested at great length, and it is found that the 
work capable of being performed by the coke alone, is 
theoretically much greater than the practical effect of the 
