Although vulcanized India-rubber is much the best material yet used for fire-pump valves, unfortunately the brass is sometimes corroded by the free sulphur contained in the rubber, so that if the pump is left standing for several weeks the rubber valve discs may become stuck to their brass seats, and, if suction has a high lift, there may not be vacuum enough to tear all the suction valves open when pump is started. 37. VALVE SEATS. _a._ All water valve seats must be of bronze composition. They may be either screwed into the deck on a taper or forced in on a smooth taper fit. With either arrangement, the seat must be either flanged out on the under side all the way round or be provided with a substantial lug opposite each rib, these lugs being expanded out after the valve is inserted. If the valve seats are not expanded after being put in place, there is a possibility that now and then a valve seat will work loose and come out, thus crippling the pump. _b._ The under side of the valve deck must be rounded over to give good bearing for the expanded part of the seat. _c._ Three-inch valves must have four or five ribs, three and a half inch valves five or six ribs, and four inch-valves six ribs. Enough ribs must be provided to give proper support to the rubber valve, but too many are objectionable, as small ports would be liable to obstruction by refuse. _d._ The edges of the valve-seat ports must be moderately rounded over, to remove such sharp edges and points, as would be liable to cut, or damage the rubber valve when under pressure. 38. VALVE STEMS. _a._ All valve stems must be of 3/4-inch Tobin bronze and of the fixed type, and must have the guard fastened on by one of the methods shown in Figs. 9 and 10. [Illustration: FIG. 9.] [Illustration: FIG. 10.] Other methods may be approved, in writing, if found by test and experience to have especial merit. _b._ These stems must be screwed into the seats on a straight, tightly fitting thread, and the lower end then well headed over into a countersink. The valve guard and nut must be of composition. In Fig. 9 the upper part of the stem is slabbed off on two opposite sides and fits a corresponding hole in the guard. The guard, therefore, cannot turn. The outside of the special nut is fitted on a taper to the inside of the guard, and the nut tapped out to fit the 5/8 U. S. thread on the stem. The action of the valve, whether with the spring or without, tends to drive these taper fits together, producing a frictional lock similar to that of a friction clutch; and although the nut may be loose on the thread, it cannot possibly work off. It will be apparent that the taper fit on the nut must be so made as to always bear on the taper fit in the guard, and not bottom in the guard.