To determine the cause of the ordinary zig-zag form of electric sparks, the author requested Mr Matheson, one of his laboratory students well skilled in photographic processes, to take instantaneous photographs of the sparks of the Holtz Machine, by means of a quartz lens, in hot and cold air alternately. Several of these photographs were exhibited, and showed much greater smoothness of the track of the spark in heated than in cold air. The zig-zag appearance seems to depend upon the presence of combustible organic particles in ordinary air, but the experiments are still in process.