description; or it may be allowed to run down a depending pipe in obedience to the law of gravitation, and stopped from running down such a pipe by opposing to its passage a gas pressure superior to that gravitational force. In any one of several obvious ways the supply of water to a mass of carbide may be controlled with absolute certainty, and therefore it should apparently follow that the make of acetylene should be under perfect control by controlling the water current. On the other hand, unless made up into balls or cartridges of some symmetrical form, calcium carbide exists in angular masses of highly irregular shape and size. Its lumps alter in shape and size directly liquid water or moisture reaches them; a loose more or loss gritty powder, or a damp cohesive mud, being produced which is well calculated to choke any narrow aperture or to jam any moving valve. It is more difficult, therefore, by mechanical agency to add a supply of carbide to a mass of water than to introduce a supply of water to a stationary mass of carbide; and far more difficult still to bring the supply of carbide under perfect control with the certainty that the movement shall begin and stop immediately the proper time arrives. But assuming the mechanical difficulties to be satisfactorily overcome, the plan of adding carbide to a stationary mass of water has several chemical advantages, first, because, however the generator be constructed, water will be in excess throughout the whole time of gas production; and secondly, because the evolution of acetylene will actually cease completely at the moment when the supply of carbide is interrupted. There is, however, one particular type of generator in which as a matter of fact the carbide is the moving constituent, viz., the "dipping" apparatus (cf. _infra_), to which these remarks do not apply; but this machine, as will be seen directly, is, illogically perhaps, but for certain good reasons, classed among the water-to-carbide apparatus. All the mechanical advantages are in favour, as just indicated, of making water the moving substance; and accordingly, when classified in the present manner, a great majority of the generators now on the markets are termed water-to-carbide apparatus. Their disadvantages are twofold, though these may be avoided or circumvented: in all types save one the carbide is in excess at the immediate place and time of decomposition; and in all types without exception the carbide in the whole of the generator is in excess, so that the phenomenon of "after- generation" occurs with more or less severity. As explained in the last chapter, after-generation is the secondary production of acetylene which takes place more or less slowly after the primary reaction is finished, proceeding either between calcium hydroxide, merely damp lime, or damp gas and calcium carbide, with an evolution of more acetylene. As it is possible, and indeed usual, to fit a holder of some capacity even to an