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It is well known that if a Leyden jar be discharged through a vacuum-tube, the discharge generally takes the form of an unbroken column of light, extending from the point of the positive terminal to the hilt of the negative,i. e. to the extreme negative end of the tube, and that it shows no trace of either negative glow or intervening dark space. On the other hand I have found, by experiments with a large Leyden battery, that if a tube have one terminal connected with the negatively charged coating of the battery and the other held beyond striking-distance from the positively charged coating, the discharge in the tube will show a separation of the positive from the negative part by a dark intervening space. Under suitable circumstances of exhaustion it will also show striae, in the same manner as when the discharge is effected directly with a Holtz machine, having the conductors either closed or open beyond striking-distance (see Roy. Soc. Proceedings, vol. xxiii. p. 460). Again, I have found, with the same battery, that if the tube be connected otherwise as before, and held at a distance less than at first, but a little greater than striking-distance, a stratified discharge much more brilliant and more like that produced by a coil will be exhibited. It should be remarked that the latter form of discharge appears to the unassisted eye, in the cases which I have examined, as an unbroken column of light, but with a negative glow and dark space. A revolving mirror, however, re­solves the column into a regular array of striae, having a rapid proper motion towards the positive terminal. The transition from the first to the second of these forms, and from the second to the jar-discharge proper when the tube was brought within striking-distance, was, if not absolutely abrupt, at all events so rapid that this form of experiment gave no prospect of following one form into the other. "With a view to examining the transition as closely as possible a Holtz machine was employed, and the jars having been taken off, a pair of mica plates partially covered with tinfoil was used in their stead. By sliding one plate over the other, so that more or less of the covered parts were brought face to face, a jar was formed the size of which could be varied at pleasure. An air-spark of adjustable length was also introduced into the circuit between the machine and the tube.

I. On stratified discharges.—IV. Stratified and unstratified forms of the jar-discharge