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ly upon the metal of the tank and bell. The water-soluble impurities in acetylene will not be removed completely by merely standing over the holder seal for a short time, and it is not good practice to pass unnecessarily impure gas into a holder. [Footnote: This is not a contradiction of what has been said in
Chapter III.
about the relative position of holder and chemical purifiers, because reference is now being made to ammonia and sulphuretted hydrogen only.]HARMFULNESS OF IMPURITIES.--The reasons why the carbide impurities must be removed from acetylene before it is burned have now to be explained. From the strictly chemical point of view there are three compounds of phosphorus, all termed phosphoretted hydrogen or phosphine: a gas, PH_3; a liquid, P_2H_4; and a solid, P_4H_2. The liquid is spontaneously inflammable in presence of air; that is to say, it catches fire of itself without the assistance of spark or flame immediately it comes in contact with atmospheric oxygen; being very volatile, it is easily carried as vapour by any permanent gas. The gaseous phosphine is not actually spontaneously inflammable at temperatures below 100° C.; but it oxidises so rapidly in air, even when somewhat diluted, that the temperature may quickly rise to the point of inflammation. In the earliest days of the acetylene industry, directly it was recognised that phosphine always accompanies crude acetylene from the generator, it was believed that unless the proportion were strictly limited by decomposing only a carbide practically free from phosphides, the crude acetylene might exhibit spontaneously inflammable properties. Lewes, indeed, has found that a sample of carbide containing 1 per cent of calcium phosphide gave (probably by local decomposition--the bulk of the phosphide suffering attack first) a spontaneously inflammable gas; but when examining spec