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Some Extinguishing Agents
Halogenated hydrocarbons. Carbon tetrachloride, CCL4, has long history as a fire-extinguishing agent. It is used in small quantities and its principal action is to supply a heavy blanket of vapour over the fire. Because it is 3,5 times as dense as carbon dioxide, it is very effective for this purpose. In addition, carbon tetrachloride, in common with all halogenated compounds, has a definite chemical inhibiting effect on combustion. even vapour percentages in the air as low as 4,5% will sometimes put out a fire. Larger percentages are usually needed, but the hologenated materials are more effective. Other halogenated hydrocarbons that have been used for fire extinguishing, are chlorobromomethane and several of fluorinated hydrocarbons known as freons. An effective agent that has been of interest is Freon 13 B-1. O ther extinguishing methods. Small fires are best treated by choking off the supply of air. For a household fire, especially involving clothing on a person, a blanket or a rug provides an effective means to smother the fire. Small fires around a kitchen stove may be put out with salt or, better, with bicarbonate of soda. A bucket of sand is also useful against domestic fire hazards. Very novel technique for putting out a fire burning at the liquid surface in an oil tank or vat is to supply agitation by blowing air at the bottom of the vessel. As a result of the agitation, the cold oil from the bottom is brought up to displace the hot burning layer of oil at the top. This method is amazingly effective. ln the case of forest fires, different techniques are used. These depend on wind conditions, moisture and fuel type. Water suspensions of sodium calcium berate have proved effective in preparing a fire break and even in direct attack on the fire. Berates are well known as fire retardants for paper and fabrics. TEXT 3
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