1881.] on the Phenomena of the Elertrie Dixrhmje. 485 



vaciiuin tube, as you will see when I coimect the first ring to one pole 

 of the battery and tlic otlicr to earth. 



We will now take up the plienoniena exhibited by vacuum tubes. 

 It will be seen that the strata have their origin at the positive pole. 

 Thus in a given tube, with a certain gas, there is produced at a 

 certain pressure, in the first instance, only one luminosity,* which 

 forms at the positive terminal ; then, as the exhaustion is gradually 

 carried further, it detaches itself, moving towards the negative, and 

 being followed by other luminosities, which gradually increase in 

 number up to a certain point. This I will show you, with Mr. Cottrcll's 

 aid, by j^rojecting copies of photographs, made in my laboratory, from 

 tubes containing hydrogen at gradually decreasing pressures. 



If I now connect the fixed terminal of the Spottiswoode tube, con- 

 taining residual carbonic acid at a pressure of 1 millimetre, with the 

 positive pole of a battery of 2400 cells, having first caused the movable 

 terminal (which I have connected previously to the negative pole) to 

 approach quite close to the positive wire, you will see only one stratum. 

 I incline the tube and allow the negative terminal to recede. Now 

 there are three strata (Fig. 10 in the plate), and as the negative recedes 

 further and further fresh strata pour in one by one from the positive 

 until the whole tube is filled to within a constant distance from the 

 negative with our electric drops (Fig. 9 in the plate). 



I may here pause to draw attention to the resemblance of the strata 

 produced by an electrical discharge in a vacuum tube to the lyco- 

 podium records of sound-pulsations in air which are given in Tyndall's 

 work on 'Sound' (Fig. 22). 



Fig. 22. 



With the same potential the phenomena vary with the amount of 

 current which is allowed to pass through the tube, and this amount we 

 can easily regulate by inserting resistances between the battery and the 

 tube. As the current is increased the number of strata in some tubes 

 increases, but with other tubes the number decreases. 



A change in the amount of current frequently produces an entire 

 change in the colour of the strata. For example, in a hydrogen tube 

 from a cobalt blue to a pink (Figs. 4 and 3 in the plate). It also 

 changes the spectra of the strata. Moreover, the spectra of the illu- 

 minated terminals and those of the strata difier; usually the most 

 brilliant spectra are obtained from the negative terminal. 



* It is not improbaLle that ball-lightning may be of the nature of this single 

 luminosity or stratum, charged like it as a Leyden jar, and projected by an electric 

 discharge taking place behind it; iu the same way that a mechanical impulse 

 sends forth a vortex ring. 



