312 M. F. Stenger on the Electric 



elements) according to Despretz *, the carbon of the electrodes 

 is changed into vapour, and on the surface of the carbon are 

 seen certain round globules molten together. With feebler 

 currents, such as are generally employed for electric lighting, 

 there can be no possibility of the volatilization of the electrodes. 

 As already remarked, all these carbons contain hydrocarbons ; 

 at the high temperature these become decomposed and leave 

 the electrode as a porous mass of carbon, which gradually 

 crumbles away. In my apparatus (fig. 2) accordingly I 

 found numerous carbon particles imbedded in the fine 

 deposit on the glass walls, which easily dissolved in nitric 

 acid, and must therefore have consisted of compounds of 

 carbon, and not of carbon itself. 



Since, farther, in a vacuum no difference of temperature 

 between the electrodes could be detected, the waste becomes 

 equal from both, disregarding, of course, differences in the 

 porosity and composition of the carbons. 



Collecting, then, the results so far obtained, we see that 

 there exists no certain criterion by which to distinguish a given 

 discharge as glow-discharge or arc-discharge, but these very 

 different typical forms pass insensibly into each other. 



Before passing from this first part to the intimately 

 connected considerations of the second, I wish to mention in 

 § 5 an observation, which appears to me to be not without 

 interest. 



§ 5. On Changes of Pressure in the Arc-light. — According to 

 Warren De La Rue and H. Miiller t, an increase of pressure 

 occurs in the arc-light upon completing the circuit, which 

 disappears immediately upon interrupting the current. 



According to their data the increase of pressure at an initial 

 pressure of 1^ to 28 millim. amounted to from 25 to 50 per cent. 

 Nevertheless some doubt remains as to the existence of the 

 phenomenon, since the experiments % hardly leave it doubtful 

 that the authors did not work with a normal arc-light. By 

 accident I made an observation which seems to show in 

 fact that the discharge produced a momentary increase in 

 pressure. I had led hydrogen into the apparatus shown in 

 fig. 4, so that the pressure amounted to about 50 millim. : as 

 soon as, by raising the mercury, contact between the carbons 

 had been established and the arc had been formed, the level 

 of mercury sank, and with it the lower carbon, until the 

 length of the arc amounted to 3 centim. Fig. 3 may 

 represent the appearance of the arc at this length. The 



* Comptes Rendus, xxviii. p. 757 (1849); xxix. pp. 48 & 709 (1849). 



t See Goldstein's criticism in Fortsch. der Physik, 1880, p. 858. 



t Phil. Trans, clxxi. p. 05 (1879) ; Troc. Key. Soc. xxix. p. 280 (1879). 



