THE SMOKE ABATEMENT EXHIBITION. 
231 
floating in the inflammable gas, and forms red, yellow, or 
WHITE flame, the flame from the fuel being larger the more 
slowly its combustion is effected. 
These appearances one is perfectly familiar with in the case 
of an ordinary fire. When the fire is fed with fresh fuel, the 
Yolatile hydrocarbons are given off in great quantity. The 
amount of air which can reach them is not sufficiently great to 
enable perfect combustion to go on, and, therefore, although a 
large amount of flame is observed, there is at the same time a 
considerable quantity of smoke produced, which w^ould not be 
produced — and this is one of the most important points to be 
borne in mind in devising means to prevent the formation of 
smoke — if a sufficient supply of air could be mixed with these 
gases whilst still at the temperature of ignition. But. after a 
time, these volatile hydrocarbons are driven off from the heated 
surface of the fuel, and the amount of air supplied being then 
sufficient to give complete combustion, first the amount of 
smoke, and then the amount of bright flame diminishes, and 
after a time the fire burns clear. But, we all know that flame 
and smoke are once more produced by poking, the explanation 
being that by poking more hydrocarbons are disengaged from 
the interior of the coal, which repeat the operations described 
above. 
III. — Besides the two great divisions of the constituents of 
fuel mentioned above, viz., carbon and the hydrocarbons, oxygen 
and hydrogen exist in coal in such quantities as by their com- 
bination to form water. The presence of this water, or of its 
constituents, in fuel, promotes the formation of smoke, or of the 
carbonaceous flame, which is ignited smoke, as the case may be, 
probably by mechanically sweeping along fine particles of carbon. 
The other constituents of coal are not important for present 
considerations, although, when the smoke is prevented from 
passing away into the atmosphere by the prevalence of fogs, 
