the Residual Gaseous Matter in Crookes Tubes. 393 



wheel in the forward position and the other with its wheel in 

 the back position. The two tubes can be operated simulta- 

 neously by connecting them in series. 



It should also be stated that for these experiments extremely 

 high vacua are requisite, and that with a Rukmkorff coil as 

 the source of electrical power the effects can only be shown 

 satisfactorily with the tube connected to the mercury-pump, 

 for the reason that the discharges from the coil inevitably 

 bring down the vacuum very quickly, apparently by their 

 action upon the mica vanes, which are visibly affected when 

 the cathode-stream from the concave cup is allowed to fall 

 upon them. Using a small Wimshurst machine, however, 

 the effects can be shown after the tube has been sealed off, 

 though even then with use the vacuum appears to deteriorate 

 in a short time. 



XXXIX. Some Further Experiments on the Circulation of the 

 Residual Gaseous Matter in Crookes Tubes. By Alan A. 

 Campbell Swinton*. 



IN the discussion which followed my former paper on this 

 subject some objection was taken to the use of a non- 

 conducting substance, i. e. mica, for the vanes of the mill 

 which was used to detect the circulation of the ultra-gaseous 

 matter, it being suggested by Professor Boys that the rotation 

 produced might be the result of electrification of the vanes. 

 It was further suggested by Mr. Appleyard that gilding the 

 vanes, so as to make them conductive, might modify the 

 effect. 



Mr. Wolff has now constructed for me a tube similar to 

 fig. 1 of my former paper, but with the mica vanes gilded and 

 mounted on a brass cap revolving upon a steel needle-point 

 connected with a wire and terminal, so that the vanes can 

 readily be earthed. 



In this condition, and with the vanes so placed as to be 

 outside the cathode-stream, it is found that (his wheel behaves 

 in a similar manner to the former non-conducting and insu- 

 lated wheel. It shows a greater tendency to assume a position 

 of stability, due evidently to electrostatic induction ; but 

 though this renders it sometimes rather troublesome in starting, 

 still when once under weigh it will continue to rotate as long 

 as the tube is excited. It will occasionally, when starting, 

 make a few reverse revolutions, due probably to electrostatic 

 influence and momentum, and also possibly to eddy-currents 



* Communicated by the Physical Society : read May 27, 1898. 



