Many experimenters have made observations on the potential difference necessary to produce electric sparks through gases. The field is a very wide one, since the number of circumstances which may be varied is large. The nature and the pressure of the gas, the shape of the electrodes, the distance between them, and the pressure of the gas, may each be altered. The investigation of which I wish to give an account in this paper deals with the potential difference required to produce sparks (or striking potential as I shall call it for brevity) in various gases, between large parallel planes, at a fixed distance apart, and at various pressures. It was found by Mr. Peace (‘Roy. Soc. Proc.,’ vol. 52, p. 99) that the striking potential between two parallel plates in air diminished as the pressure diminished till a certain point was reached, and then began to rise very rapidly. The pressure at which the striking potential was a minimum depended on the distance between the plates, and increased as the distance was lessened. The minimum potential itself, however, varied very little with the distance between the plates.