Conductivity of Gases. 307 



was strongly heated, no change in the current-strength could 

 be recognized. 



Goldstein has obtained exactly similar results. It appears 

 therefore from these investigations that it is only necessary to 

 employ suitable experimental conditions, in order to obtain 

 with the glow-discharge currents of the same order of magni- 

 tude as with the arc-light. 



§ 2. Does the temperature of the electrodes in the arc- 

 discharge depend upon the pressure and nature of the 

 surrounding gas ? 



Gassiot was the first to make observations on the different 

 temperature of the electrodes. A few years later Grove * 

 investigated the behaviour of the arc-light in different gases, 

 and in a vacuum such as could be obtained at that time. His 

 results only partially agree with my observations. According 

 to Grove, the temperature of both electrodes is the same in 

 hydrogen or in nitrogen, or in a tolerably perfect vacuum, 

 judging from the colour and rapidity of cooling. But, as he 

 himself remarks, in an atmosphere of hydrogen he only 

 succeeded with carbon electrodes in maintaining the arc 

 constant for any time, so that his result that then the electrodes 

 show no difference in temperature cannot be considered of 

 any great weight. I have also endeavoured in vain to obtain 

 a constant arc-light between metallic electrodes in an atmo- 

 sphere of hydrogen. As Liveing and Dewarf have remarked, 

 the length of the arc is much less in hydrogen than in air, and 

 hence a small increase in the distance of the electrodes apart 

 extinguishes the light. But that the temperature of the 

 electrodes may be very greatly different, when the current 

 is closed only for a short time, and when it is maintained 

 burning, appears clearly from the statement of Moigno, that 

 after contact between the electrodes has been broken for pro- 

 duction of the arc, first white light flashes out from the point 

 of the negative electrode, and only then does the positive 

 begin to glow. With carbon electrodes, on the other hand, I 

 have established by a series of experiments that in hydrogen, 

 as in nitrogen, the anode always possesses a higher temperature 

 than the kathode, although the difference may not be so 

 marked as in air. There would appear then to be some 

 ground for Grove's suggestion that secondary phenomena play 

 a part in gases which contain oxygen. That in observations 

 of this kind the arc should be maintained for some minutes, 

 before making the comparison of temperatures, appears from 

 the fact that both carbons are often seen to cool simultaneously, 



* Phil. Mag. [3] xvi. p. 478 (1840). 

 f Proc. Roy. Soc. xxx. p. 156 (1880). 

 Z2 



