under the Influence of the Electric Discharge. 417 



question whether we have to do here with discontinuous or 

 continuous discharge. 



The current from a Hittorf s cell, when newly put toge- 

 ther, will liberate a maximum of 90'2 milligrammes of silver 

 in a minute ; so that this would in general represent its 

 maximum strength of current*. 



The current of my Topler's machine corresponded to 0*0006 

 Siemens-Daniell for a deflection of the galvanometer of 100 

 millhns., and would therefore separate about 77 x 0*0004 milli- 

 gramme of silver. It is therefore == — j or about 3000 



times as weak as that employed by Hittorf. But with this 

 current I obtained, with electrodes widely separated by capil- 

 lary tubes, about 2000 discharges in a second at a pressure of 

 about 5 millims. If we assume that Hittorf s battery would 

 furnish the same quantity of electricity as given above when 

 used with the discharge-tube, we should have 3000 x 2000 = 

 6,000,000 discharges in the second. If the part of the tube 

 seen in the revolving mirror have a breadth of 1 millim. and 

 the mirror make 100 revolutions per second, then, when 

 the observer and the tube are each at a distance of half a 

 metre from the mirror, the image would appear to move 

 1000 x 100 x 2 x 3*14, or 600,000 millims. in a second. Ac- 

 cordingly, six separate discharges would appear to fall upon 

 each millimetre of the image. But as the impression of the 

 light on the retina lasts more than one seventh of a second, we 

 see the images corresponding to 14 revolutions of the mirror at 

 the same time ; so that the images of 126 separate discharges 

 are seen together in each millimetre. 



In the tubes employed by Hittorf the electrodes were much 

 nearer together, and the pressures were lower; so that the 

 number of discharges would be further considerably increased. 



With so large a number of discharges the heating and lumi- 

 nosity of the gas does not sink again to nothing between each 

 two discharges ; so that the separate images appear length- 

 ened out in the mirror. Hence it is here quite impossible to 

 draw conclusions about continuity or discontinuity of dis- 

 charges. Moreover the criterion given by Hittorf to decide 

 the question of continuity — the want of sensitiveness of the 

 discharge on approach of the Auger — is not always applicable, 

 as experiments with the Topler's machine have shown that the 

 discontinuous discharges are almost altogether non-sensitive; 

 but they can be made more or less sensitive by the interpola- 

 tion of air-sparks of definite length. Most probably, in most 



* Hittorf, Wied. Ann. vii. p. 557 (1880). 



