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has been explained in a previous chapter how the reciprocating piston takes in a charge of explosive mixture and compresses it. It is now necessary to ignite this compressed gas in some manner, in order that an explosion may take place and drive back the piston with great force.

There are two methods of accomplishing this in the petroleum spirit motor, (a) by means of a hot platinum tube, which is known as 'tube ignition'; (b) by an electric spark, known as 'electrical ignition.'

The Platinum Tube.—At a convenient place in the wall of the explosion chamber a hole is drilled and means provided for attaching about it a nut, in the course of which is drilled a hole of sufficient size to take a platinum tube, closed at one end and provided at the other extremity with a flange. This flange must have a perfectly true face, and fit accurately against the face of the hole in the explosion chamber. Between the flange and the top of the nut is placed a thin asbestos washer; the nut being screwed up, the flange on the platinum tube is held tightly against the hole, so that any gas in the cylinder may have free access the whole way up the platinum tube, but none can escape at the joints.