At the end of the chamber opposite the doors, a pane of ¼ in. wire plate glass, 36 × 48 in., is set into the wall, and additional illumination may be secured by 2 headlights, 12 in. square, set into the ceiling of the chamber and of the air-lock, respectively, and provided with 200 watt Mazda lamps and Holophane reflectors. Openings into the chamber, five in number, are spaced across this end beneath the window and 9 in. above the table top. Fans are installed for keeping the concentration uniform. [Illustration: FIG. 77.—Details of Canister Holder.] Various devices have been installed for attaching the canister to be tested (Fig. 77). This arrangement allows the canister to be changed at will without any necessity for disturbing the concentration of gas by entering the chamber. Arrangements for removing the gas from the chamber consist of a small “bleeder” which allows a continuous escape of small amounts and a large blower for rapidly exhausting the entire contents of the chamber. Other general features of the equipment deal with the determination of the physical condition surrounding the tests, often a matter of considerable importance. The temperature of the gas inside the chamber is easily ascertained by means of a thermometer suspended inside the window in such a position as to be read from the outside. The relative humidity of the mixture of air and gas in the chamber is determined by means of a somewhat modified Regnault dew point apparatus mounted on the built-in table. PRESSURE DROP AND LEAK DETECTING APPARATUS Another piece of apparatus consists of a combined pressure drop machine and leak tester (Fig. 78) for measuring the resistance of canisters and testing them for faulty construction. This is mounted on a small table, with the motor and air pump installed on a shelf underneath. The resistance, or pressure drop, of canisters is measured by the flow meter _A_ and the water manometer _B_. Air is drawn through the canister and the flow meter _A_ at the rate of 85 liters per min., the flow being adjusted by the needle valve. The pressure drop across the canister is read on the water manometer _B_, one end of which is connected to the suction line, the other open to the air. The reading is generally made in inches, correction being made for the resistance of the connecting hose and the apparatus itself.