280 
THE SMOKE ABATEMENT EXHIBITION. 
Before I pass on to the description of the various appliances 
lately exhibited at South Kensington to prevent smoke, I may 
shortly recall to your remembrance the conditions on which 
perfect combustion depends. Combustion or burning is a rapid 
chemical combination of two or more substances. The only 
kind of combustion that concerns us now is that in which the 
various combustibles contained in coal combine with oxygen so 
as to produce light and heat. '1 he ingredients of every kind of 
fuel commonly used may be divided into three classes. 
I. — Fixed and free carbon, which is left in the form of charcoal 
(from w’ood) or coke (from coal) when the volatile constituents of 
the coal have been burned away. Carbon can burn either in the 
solid or gaseous state, or partly in the one and partly in the 
other. 
II. — Hydrocarbons, such as olefiant gas, pitch, tar, naphtha, 
(Ic., which must be converted into the gaseous state before they 
are burned. 
If these gases, when they first issue from the burning fuel, 
come in contact with a larger quantity of oxygen, they burn 
completely with a pale transparent blue flame, producing 
carbonic acid and steam. When raised to a red heat, or 
thereabouts, before being mixed with a sufficient quantity of air 
the carbon which they contain is separated in the form of fine 
powder, and the gases pass into the state either of marsh gas or 
free hydrogen — the higher the temperature the greater is the 
amount of carbon so separated. 
If this disengaged carbon, instead of being burnt by coming 
in contact with the requisite amount of air at the high 
temperature at which it is separated, is cooled below the 
temperature necessary for ignition, it constitutes, while floating 
in the air, smoke, and when deposited on bodies, it forms soot. 
But if the carbon powder comes in contact with air before it 
has been cooled below its point of ignition, it burns while 
