retention of air in the chamber. ACTION OF CARBIDE-TO-WATER GENERATORS.--The methods which may be adopted to render a generator automatic when carbide is employed as the moving material are shown at M, N, and P, in Fig. 6; but the precise devices used in many actual apparatus are so various that it is difficult to portray them generically. Moreover it is desirable to subdivide automatic carbide-to-water generators, according to the size of the carbide they are constructed to take, into two or three classes, which are termed respectively "large carbide-feed," "small carbide-feed," and "granulated carbide-feed" apparatus. (The generator represented at L does not really belong to the present class, being non-automatic and fed by hand; but the sketch is given for completeness.) M is an automatic carbide-feed generator having its store of carbide in a hopper carried by the rising- holder bell. The hopper is narrowed at its mouth, where it is closed by a conical or mushroom valve _d_ supported on a rod held in suitable guides. When the bell falls by consumption of gas, it carries the valve and rod with it; but eventually the button at the base of _c_ strikes the bottom of the generator, or some fixed distributing plate, and the rod can descend no further. Then, when the bell falls lower, the mushroom _d_ rises from its seat, and carbide drops from the hopper into the water. This type of apparatus has the defect characteristic of A^2, Fig. 1; for the pressure in the service steadily diminishes as the effective weight of bell plus hopper decreases by consumption of carbide. But it has also two other defects--(1) that ordinary carbide is too irregular in shape to fall smoothly through the narrow annular space between the valve and its seat; (2) that water vapour penetrates into the hopper, and liberates some gas there, while it attacks the lumps of carbide at the orifice, producing dust or causing them to stick together, and thus rendering the action of the feed worse than ever. Most of these defects can be avoided by using granulated carbide, which is more uniform in size and shape, or by employing a granulated and "treated" carbide which has been dipped in some non-aqueous liquid to make it less susceptible to the action of moisture. Both these plans, however, are expensive to adopt; first, because of the actual cost of granulating or "treating" the carbide; secondly, because the carbide deteriorates in gas-making capacity by its inevitable exposure to air during the granulating or "treating" process. The defects of irregularity of pressure and possible waste of gas by evolution in the hopper may be overcome by disposing the parts somewhat differently; making the holder an annulus round the hopper, or making it cylindrical with the hopper inside. In this case the hopper is supported by the main portion of the apparatus, and does not move with the bell: the rod and valve being given their motion in some fashion similar to that figured.