232 Royal Society : — Mr. W. Spottiswoode on 



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 inter- 

 vening dark space. On the other hand I have found, by experi- 

 ments with a large Leyden battery, that if a tube have one terminal 

 connected with the negativel}^ charged coating of the battery and 

 the other held beyond striking-distance from tlbe 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 \\dll 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 Hoy. 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 1 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. 



This arrangement was subsequently replaced by the following, 

 which in some respects proved more convenient : — A battery of 

 one or more jars was used in the place of the mica plates. The 

 outside of this battery and one terminal of the tube were connected 

 with the earth ; and the inside and the other terminal were al- 

 ternately connected with the positive conductor of the machiiae, 

 so that the battery was alternately charged and discharged through 

 the tube. The amount of charge was regulated partly by the dis- 

 tance through which the conductors of the machine were separated, 

 and partly by the number of revolutions of the machine during 

 which the charging took place. It was consequently independent 

 of the absolute time of contact. It wdll be observed that this ar- 

 rangement did not give the same opportunity of a continuous varia- 

 tion of jar surface as the first ; but, on the other hand, the changes 

 of phase in the phenomenon due to increments of charge were 

 capable of indefinite diminution by shortening the distance between 



