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fuels in common use, we are now in a position to examine 
the effects of heat upon them. All fuels are of very com- 
plex constitution, and it is a law of organic chemistry that 
the greater the complexity of a body, the easier is its 
decomposition effected. In bodies so compounded the effect 
of decomposition is usually a disturbance of the existing 
equilibrium of the elements, and their re-arrangement into 
new compounds. The nature of these compounds depends 
on the external circumstances with which the decomposition 
is attended, and on the presence or absence of oxygen. 
The action of heat upon fuel exhibits instances of decompo- 
sition of both kinds. The first process is a dry distillation 
of the elements of the coal without access to air. It is 
erroneous to suppose that the combustion of fuel is caused 
by the direct action of atmospheric oxygen on the elements 
themselves. The hydrogen and oxygen of the fuel, on the 
application of heat, abandon their former state of equilibrium, 
and form new products with the carbon, the superior affinity 
of which prevents the formation of more water than the 
equivalent of oxygen and hydrogen in the coal admits. We 
have, therefore, first, the oxygen uniting with the hydrogen 
to form water, and then the excess of hydrogen (which 
there always is in coal above the quantity required to form 
water with the oxygen) unites partly with the nitrogen, and 
forms (NH^) ammonia, and partly with the carbon, whose 
equilibrium has been disturbed by the movements of the 
other elements ; carburetted hydrogen ^4) and olefiant 
gas H^) resulting. These binary compounds again 
acting chemically on each other, give rise to an endless 
series of ternary compounds of small economic value, of 
which the principal are acetic acid, pyroxylic or a wood 
spirit, naphtha, naphthaline, paraffin, picamar, kreosot, 
kopnomor, and pitacall. The second process is the com- 
bustion of these products of distillation, and it is this 
