FILTERS.--It has just been stated that the purifying system added to an acetylene installation should not be called upon to remove these generator impurities; because their appearance in quantity indicates a faulty generator, which should be replaced by one of better action. On the contrary, with the exception of the gases which are permanent at atmospheric temperature--hydrogen, carbon monoxide, nitrogen, and oxygen-- and which, once produced, must remain in the acetylene (lowering its illuminating value, but giving no further trouble), extraction of these generator impurities is quite simple. The dust or froth of lime will be removed in the washer where the acetylene bubbles through water--the dust itself can be extracted by merely filtering the gas through cotton-wool, felt, or the like. The least volatile liquid impurities will be removed partly in the condenser, partly in the washer, and partly by the mechanical dry-scrubbing action of the solid purifying material in the chemical purifier. To some extent the more volatile liquid bodies will be removed similarly; but a complete extraction of them demands the employment of some special washing apparatus in which the crude acetylene is compelled to bubble (in finely divided streams) through a layer of some non-volatile oil, heavy mineral lubricating oil, &c.; for though soluble in such oil, the liquid impurities are not soluble in, nor do they mix with, water; and since they are held in the acetylene as vapours, a simple passage through water, or through water-cooled pipes, does not suffice for their recovery. It will be seen that a sufficient removal of these generator impurities need throw no appreciable extra labour upon the consumer of acetylene, for he can readily select a type of generator in which their production is reduced to a minimum; while a cotton-wool or coke filter for the gas, a water washer, which is always useful in the plant if only employed as a non-return valve between the generator and the holder, and the indispensable chemical purifiers, will take out of the acetylene all the remaining generator impurities which need, and can, be extracted.