1 cylinder (used alone) 2 pressure tanks 2 cylinders (in combination) 6 pressure tanks 4 cylinders (in combination) 24 pressure tanks 5 cylinders (in combination) 36 pressure tanks 6 cylinders (in combination) 48 pressure tanks (3) _Apparatus._ The apparatus for charging two pressure tanks by the use of cylinders consists of a filling line, two charging lines, and two cylinders. (Fig 23) The filling line and charging lines are obtained from the service kit. (Par 48) Plugs are provided to close off either half of the filling line when only one flame-thrower tank group is to be charged. (4) _Warning._ Oxygen is sometimes shipped in cylinders having the same threads as nitrogen cylinders. If oxygen not mixed with nitrogen, as in air, is introduced into the fuel tanks of the portable flame thrower, a violent explosion may result. Therefore, the greatest care must be exercised to see that only air or nitrogen is used. _Before a cylinder is connected, it should be tested to determine that it does not contain straight oxygen or some combustible gas._ This may be done by introducing a burning splint into a jet of the contents. Oxygen causes the splint to burn quickly, whereas nitrogen extinguishes the flame. To make the test: (_a_) Fasten a thin splint of wood to a wire at least a foot long. (_b_) Ignite the splint. (_c_) Stand aside and hold it before the cylinder outlet. (_d_) Crack the valve slightly to permit a small stream of gas to emerge. (_e_) If the flame flares up, the gas is oxygen and _MUST NOT_ be used. (_f_) If the gas itself catches fire, it may be hydrogen, acetylene, or some other combustible gas, which also must not be used. (5) _Attaching lines to cylinders._ (Fig 23) The procedure for charging two flame thrower pressure tanks from two cylinders of nitrogen or compressed air begins as follows: (_a_) Remove the valve-protection caps from the cylinders. [Illustration: Fig 23. Charging two pressure tanks, using charging and filling lines, and cylinders of compressed air or nitrogen.] (_b_) Place the cylinders side by side with both outlets facing in the same direction. (If the ground is not level enough for the cylinders to stand up side by side, lay them horizontally with both outlets face up.) (_c_) Before attaching the filling line to the cylinders, blow out dust. (Par 33) Then connect, using wrenches to make the joints pressure tight. Do not kink or bend the flexible hose. Cylinders must be close enough together to prevent strain on the flexible hose. (_d_) Attach a charging line to each of the two couplings on the filling line. (6) _Attaching charging lines to pressure tanks._ (_a_) Close pressure-tank valves. (_b_) Unscrew caps from check valves. (_c_) Screw the charging-line fittings onto the check valves. (_d_) Close bleeders. (7) _Charging._ The operation of charging two pressure tanks from two cylinders is as follows: (_a_) Close both filling-line valves.