332 Dr. B. Hasselberg on the 



unite in each discharge, the mechanical equivalent of the 

 discharge is greater, and consequently its temperature is 

 higher. The expansion of the lines is therefore to be re- 

 garded only as the consequence of this rise in temperature *; 

 and if the same effect is produced by increase of pressure, 

 this is only because then a higher potential is necessary in 

 order that the discharge shall take place. 



If, instead of the induction-coil, we employ an influence- 

 machine as source of electricity, then, as E. Wiedemann! has 

 shown, the simpler process of discharge which takes place 

 with the machine permits the two spectra to be much more 

 clearly separated from each other, by gradually lengthening 

 the included air-break, than is possible with the more coin- 

 plicated process of discharge in the induction-coil. The 

 second spectrum, which appears with no air-break, or only a 

 small one, becomes gradually less bright as the air-break is 

 increased, whilst the characteristic lines become more distinct, 

 until finally, with a certain length of spark, the second spec- 

 trum disappears almost suddenly. These conditions are 

 exactly suited to make the phenomena obtained with the 

 induction-coil more intelligible. Since, on account of the 

 gradual flow of electricity to the electrodes which takes place, 

 each opening or closing of the primary current, besides the 

 chief discharge, produces also a whole series of feebler partial 

 discharges, it is probable that the second spectrum is duo 

 to these smaller discharges, whilst the primary discharge at 

 greater potential produces the characteristic lines. Conse- 

 quently, in more completely exhausted tubes, where the resist- 

 ance is not sufficient to prevent the passage of the smaller dis- 

 charges, both spectra should be visible; whilst as the pressure 

 is increased a point is at last reached at which only fcne prin- 

 cipal discharge takes place, and the spectrum due to it, con- 

 sisting of the lines H a . . . H 5 , is seen. If this is so, then in 

 one and the same tube, in which the exhaustion is gradually 

 increased, when currents of different strengths are employee!, 

 that pressure at which the second spectrum begins to show 

 itself must be greater the greater the intensity of the current, 

 and consequently the potential of the partial discharges. But 

 this is confirmed by experiments Tims, for example, on 

 passing the current from a large induction-coil excited by 

 means of fire Bunsen cells througn a tube containing hydrogen, 

 and then passing the current of a small induction-coil througn 

 it, I found that on gradually exhausting, the first traces of 



* Compare Salet, Sur lei spectres des MitaMoldes (Paris L872), ]>. \7 , 

 Fierez, Hull, de VAcade'mie de Belgique f 8 b6t. i. I, No. 8, 1881, 



f Wiedemann's Annalcn, vol. x. 1880, p. 202. 



