CARBIDE IMPURITIES.--Neglecting very minute amounts of carbon monoxide and hydrogen (which may perhaps come from cavities in the calcium carbide itself), as being utterly insignificant from the practical point of view, the carbide impurities of the gas fall into four main categories: those containing phosphorus, those containing sulphur, those containing silicon, and those containing gaseous ammonia. The phosphorus in the gas comes from calcium phosphide in the calcium carbide, which is attacked by water, and yields phosphoretted hydrogen (or phosphine, as it will be termed hereafter). The calcium phosphide, in its turn, is produced in the electric furnace by the action of the coke upon the phosphorus in phosphatic lime--all commercially procurable lime and some varieties of coke (or charcoal) containing phosphates to a larger or smaller extent. The sulphur in the gas comes from aluminium sulphide in the carbide, which is produced in the electric furnace by the interaction of impurities containing aluminium and sulphur (clay-like bodies, &c.) present in the lime and coke; this aluminium sulphide is attacked by water and yields sulphuretted hydrogen. Even in the absence of aluminium compounds, sulphuretted hydrogen may be found in the gases of an acetylene generator; here it probably arises from calcium sulphide, for although the latter is not decomposed by water, it gradually changes in water into calcium sulphydrate, which appears to suffer decomposition. When it exists in the gas the silicon is derived from certain silicides in the carbide; but this impurity will be dealt with by itself in a later paragraph. The ammonia arises from the action of the water upon magnesium, aluminium, or possibly calcium nitride in the calcium carbide, which are bodies also produced in the electric furnace or as the carbide is cooling. In the gas itself the ammonia exists as such; the phosphorus exists mainly as phosphine, partly as certain organic compounds containing phosphorus, the exact chemical nature of which has not yet been fully ascertained; the sulphur exists partly as sulphuretted hydrogen and partly as organic compounds analogous, in all probability, to those of phosphorus, among which Caro has found oil of mustard, and certain bodies that he regards as mercaptans. [Footnote: It will be convenient to borrow the phrase used in the coal-gas industry, calling the compounds of phosphorus other than phosphine "phosphorus compounds," and the compounds of sulphur other than sulphuretted hydrogen "sulphur compounds." The "sulphur compounds" of coal-gas, however, consist mainly of carbon bisulphide, which is certainly not the chief "sulphur compound" in acetylene, even if present to any appreciable extent.] The precise way in which these organic bodies are formed from the phosphides and sulphides of calcium carbide is not thoroughly understood; but the system of generation employed, and the temperature obtaining in the apparatus,