Any ordinary combustion process is usually initiated by decomposition of the combustible material as a result of heating, which is usually confined to small local areas and which may be due to friction, irradiation, and chemical oxidation, action of sparks or flames. This decomposition produces combustible gases which ignite because they react violently with the oxygen of the air. The reaction produces combustible gases which ignite because they react violently with the oxygen of the air. The reaction produces heat which causes further decomposition of the material and thus produces more gas. Eventually the temperature rises so high that the residue from the initial decomposition due to heat also begins to burn, producing a large amount of heat in the process. This continuously liberated heat keeps the combustion process going until the combustible material, or the oxygen needed to sustain the process, has been consumed.

Fire extinguishing agents must therefore either cool the combustible material, or they must cover this material with a firmly adhering non-inflammable coating, or they must rarify or displace the oxygen from the focus of the outbreak. As there are, for example, combustible substances which themselves contain the oxygen necessary to sustain combustion, and as it is of considerable importance - from the point of view of fire-fighting - whether it is lighter of heavier than the fire extinguishing agent, there must obviously be different kinds of extinguishing agents and different fire-fighting methods.

The carbon tetrachloride extinguisher contains anything from 0.5 to 6 liters of this chemical, which is forced out of the extinguisher by the gas pressure from a cylinder of liquefied carbon dioxide. Carbon tetrachloride vaporizes completely at 76.5°C, producing a heavy incombustible vapor. This kind of extinguisher is more particularly suitable for putting out fires in machine and electrical installations. The carbon dioxide extinguisher contains 5 to 6 liters of carbon dioxide under high pressure. It is expanded to atmospheric pressure in the snow tube, where the greater part rapidly vaporizes and, in doing so, extracts so much heat from the surrounding that the rest of the carbon dioxide is cooled to solid carbon dioxide snow. This snow is sprayed on to the fire by the carbon dioxide gas and causes a lowering of the temperature below the ignition point; it also displaces oxygen. This is an all-purpose extinguisher.

The water type fire extinguisher contains 6 to 12 liters of water containing dissolved sodium bicarbonate. When the pin is struck, it shatters a flask of concentrated sulphuric acid inside the extinguisher. The acid reacts with the sodium bicarbonate, whereby a large quantity of carbon dioxide is evolved which forces the water at high pressure out of discharge pipe. Because of its high heat of evaporation, water exerts a powerful cooling action; besides the water vapor displaces lighting a fire in electrical installations or when inflammable solvents catch fire.

The dry chemical extinguisher contains 4 to 12 kg of sodium bicarbonate which is hurled into the blaze of gas pressure developed by liquefied carbon dioxide or nitrogen. In the fire the sodium bicarbonate is decomposed int0 sodium dioxide or nitrogen. In the fire the sodium bicarbonate is decomposed into sodium and into water vapor and carbon dioxide. The application of this type of extinguisher will depend upon the nature and place of the fire. In the larger foam type fire extinguishers and air or nitrogen filled foam is produced. The foaming agent may, for example, consist of a decomposed protein substance. The foam is stabilized by the admixture of urea, plastics etc. it exercises a smothering and cooling action on the fire. Foam extinguishers can be used for any fire fighting purpose, other than for putting out fires in machinery and electrical equipment.