There are, of course, some impurities in the purified gas. These are compounds containing sulphur and ammonia. The quantities of these substances, however, are so small that they are harmless; but the compounds taken out in the process of purifying the gas are saved, as considerable use is made of them. The water used for washing the gas is heavily charged with ammonia and is, in fact, the chief source of the ammonia sold by druggists. [Illustration: HOW THE IMPURITIES ARE TAKEN FROM THE GAS PURIFYING BOXES. The principal impurity to be removed is sulphur, and this is accomplished by passing the gas through large iron rectangular boxes filled with wood shavings coated with iron rust upon which the sulphur is deposited by chemical action.] [Illustration: STATION METER HOUSE, SHOWING CONSTRUCTION OF TWO NEW 13-FT. METERS.] [Illustration: HOW THE METER MEASURES THE GAS Fig 1 Fig 3 Fig. 2. Fig 4 Gas first enters inlet pipe _A_ (Fig. 3) passing along _A1_ into covered valve chamber _B_ up through orifice _O_. It then passes down through two of the valve ports at the same time, ports _C_ and _D1_ (Fig. 2). Before _C1_ (Fig. 3) has gotten to its extreme opening, the valve on the opposite side has moved to allow gas to pass down port _D_. On every quarter turn of tangent _P_, one port is opening to receive gas which passes down through the valve ports into the chambers below (see arrows on Fig. 2), which shows the gas passing into chamber _F_. The pressure being greater on the outside of the diaphragm, forces the diaphragm inward and expels the gas from the inside of _D2_ through _D_ and passes over the cross-bar into the fork channel (see Fig. 1). On the other side gas is passing down through port _D1_ (Fig. 2) entering diaphragm _D3_, the pressure being greater on the inside of _D3_ therefore forces the diaphragm outward and expels the gas from the outside of diaphragm _D3_; out through port _C1_ into fork channel same as shown in (Fig. 1). All exhaust gas from the chambers below is checked from entering the chamber _B_ by the slide valve _G_ and _G1_ (Fig. 2). Instead of passing into chamber _B_ it passes over the cross-bars between _D1E1_ and _C1E1_ into the fork channels, then to outlet pipe _N_ (Fig. 3) to house pipe. NOTE: All gas registered must pass through outlet _N_.]