434 ^r. Spottiswoode [May 21, 



terminal in the form of a cone set transversely to the axis. If the 

 tube be then touched at a point opposite to the cone, a patch of relief 

 phosphorescence will be formed round the root of the terminal in 

 question ; but the patch will have a dark circular centre, due to the 

 action of the cone as a shield against the molecular streams. If, 

 however, the terminal itself be at the same time touched by a con- 

 ductor, it also will shed molecular relief streams, which will interfere 

 with those first mentioned, and will greatly increase the size of the 

 circular dark patch. 



We now come to the ultimate question that we have proposed for 

 this evening, viz. the small time-quantities involved in the discharge. 

 And, in the first place, it must be understood that the whole duration 

 of the visible discharge is comprised within a period of which the 

 most rapidly revolving mirror has been incomj^etent to give any 

 account. It may be in the recollection of some of my audience that 

 when the discharge from my great Induction coil was exhibited in 

 this theatre with tubes on a revolving disk, the discharge showed a 

 durational character as long as the coil alone was used ; but as soon 

 as a Leyden jar was introduced, which was in the main equivalent to 

 an air-spark in a continuous current, the durational character dis- 

 appeared, and nothing was visible but a bright line, the width of 

 which depended, not upon the duration of the discharge, for no velo- 

 city of rotation in any way affected it, but only on the width of the 

 slit through which the discharge in the tube was seen. But, notwith- 

 standing the extreme rapidity with which the discharge is efiected, 

 our experiments have already shown that the spark or discharge is a 

 complicated phenomenon, the various parts of which take place in a 

 certain order or sequence of time ; and that in virtue of this sequence 

 we have succeeded, at the various pressures comprised within our 

 range, in attecting and modifying it in transitu. This suggested the 

 idea that, although the subject is surrounded with difficulties, it 

 might still be possible to form some relative estimate, at all events, 

 of the time occupied by the various parts of which the whole pheno- 

 menon is composed. And in fulfilment of this, the following are 

 some of the conclusions to which we have been led. 



The time occupied in the passage of electricity of either name along the 

 tube is greater than that occupied in its passage along an equal length of wire. 



This may be shown by connecting metallically a piece of tinfoil 

 near the air-spark terminal with another near the distant terminal ; 

 for it is then seen that the former derives as much relief as if the 

 latter were not on the tube. This shows (1) that at the time when 

 the electric disturbance reached the nearer piece of tinfoil, the more 

 distant piece was unaffected, and (2) that the disturbance propagated 

 along the wire reached the second piece before the arrival of the same 

 disturbance propagated within the tube. 



The negative discharge occupies a period greater than that required by 

 the particles composing the molecular streams to traverse the length of the 

 tubcj but comparable with it. 



