What has been said in the preceding paragraph about the temperature at which acetylene decomposes is only true when the gas is free from any notable quantity of air. In presence of air, acetylene inflames at a much lower temperature, viz., 480° C. In a manner precisely similar to that of all other combustible gases, if a stream of acetylene issues into the atmosphere, as from the orifices of a burner, the gas catches fire and burns quietly directly any substance having a temperature of 480° C. or upwards is brought near it; but if acetylene in bulk is mixed with the necessary quantity of air to support combustion, and any object exceeding 480° C. in temperature comes in contact with it, the oxidation of the hydrocarbon proceeds at such a high rate of speed as to be termed an explosion. The proportion of air needed to support combustion varies with every combustible material within known limits (_cf._ Chapter VI.), and according to Eitner the smallest quantity of air required to make acetylene burn or explode, as the case may be, is 25 per cent. If, by ignorant design or by careless manipulation, the first batches of acetylene evolved from a freshly charged generator should contain more than 25 per cent. of air; or if in the inauguration of a new installation the air should not be swept out of the pipes, and the first makes of gas should become diluted with 25 to 50 per cent. of air, any glowing body whose temperature exceeds 480° C. will fire the gas; and, as in the former instance, the flame will extend all through the mass of acetylene with disastrous violence and at enormous speed unless the gas is stored in narrow pipes of extremely small diameter. Three practical lessons are to be learnt from this circumstance: first, tobacco-smoking must never be permitted in any building where an escape of raw acetylene is possible, because the temperature of a lighted cigar, &c., exceeds 480° C.; secondly, a light must never be applied to a pipe delivering acetylene until a proper acetylene burner has been screwed into the aperture; thirdly, if any appreciable amount of acetylene is present in the air, no operation should be performed upon any portion of an acetylene plant which involves such processes as scraping or chipping with the aid of a steel tool or shovel. If, for example, the iron or stoneware sludge-pipe is choked, or the interior of the dismantled generator is blocked, and attempts are made to remove the obstruction with a hard steel tool, a spark is very likely to be formed which, granting the existence of sufficient acetylene in the air, is perfectly able to fire the gas. For all such purposes wooden implements only are best employed; but the remark does not apply to the hand-charging of a carbide-to-water generator through its proper shoot. Before passing to another subject, it may be remarked that a quantity of air far less than that which causes acetylene to become dangerous is objectionable, as its presence is apt to