530 PRACTICAL ORGANIC CHEMISTRY fire, when the "blanketing" effect of the heavy carbon tetrachloride vapour will quickly extinguish the fire. In such cases it should be remembered, however, (a) not to use carbon tetrachloride if metallic sodium or potassium is present, as violent explosions may result, (b) to ventilate the laboratory immediately after extinguishing the fire, in order to disperse the phosgene vapour which is always formed when carbon tetrachloride is used in this way. (3) Carbon dioxide. The above disadvantages of carbon tetra-chloride have caused it to be now superseded b}- carbon dioxide, which is available in 2.1 Ib. (1-13 Kgm.) and 5 Ib. (2-26 Kgm.) extinguishers; these are the most effective extinguishers for use in the laboratory. Carbon dioxide has the advantages that in use (a) adverse chemical reactions are extremely unlikely, (b) there is no electrical hazard, and (c) damage to apparatus is minimal. (4) If a liquid which is being heated in a beaker or a conical flask
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