purifying process requires only a washer or scrubber. From the evaporator, the cooled gas passes through a vent valve j into the bottom of the scrubber c, and up through a bed of coke k, over which water is sprayed continuously. While passing through this wet coke, the gas is further cooled and cleansed of any fine ash or foreign matter it may have carried over from the generator. The gas leaving the scrubber descends through a vertical pipe l and enters the sawdust purifier d, within which it rises to the top through a short vertical pipe, and thence filters down through several layers m, m of sawdust supported on wooden gratings. The 'size of this purifier cylinder is such as to allow the gas to travel very slowly, and any impurities, such as tar, moisture, and dust, not extracted in the scrubber are caught in the sawdust beds. The comparatively large volume of the purifier, which serves as a receiver, reduces the pulsating effect due to the engine and makes the suction at the generator practically constant.
76. The plant is started by first building a fire on the grate of the generator, and charging it with coke or coal, the chimney pipe being open and the engine gas pipe closed. Air is drawn through the generator by means of a small hand blower n until the charge is well ignited, the vent valve j being turned so that the smoke will escape to the atmosphere through the pipe o.
The fuel having become incandescent up to the bottom of the hopper, the passage to the chimney is closed and the gas is tested by igniting a small jet from a test cock. If the gas burns with a reddish-blue flame not easily extinguished, the valve leading to the engine may be opened and the engine started. As soon as the engine has made a few revolutions it will pump its own gas. The coal is fed to the hopper every half-hour or hour, depending on the size of the plant.
77. When the plant is to be shut down, the generator is filled with coal and the vent valve to the chimney opened to admit just enough air to maintain a fire. Opening the vent valve to the chimney closes communication with the scrubber,
