310 M. F. Stenger on the Electric 



the lines C and F in the arc produced by the alternating 

 current of a De Meritens machine in an atmosphere of 

 hydrogen ; but with the continuous current of a Siemens 

 machine, on the other hand, only the line C was observed at the 

 moment of interrupting the current, and the F line not even 

 then distinctly. The reason of this behaviour is to be found 

 only in the overpowering brilliancy of the spectrum of the 

 white-hot particles of carbon ; for in my experiments in vacuo 

 the hydrogen lines as well as the metallic lines stood out extra- 

 ordinarily sharply from the bright background of the arc 

 just the same, whether the apparatus had been filled with 

 hydrogen or with dry air. If, on the other hand, we observe 

 the light emitted by the electrodes themselves, the phenomena 

 are essentially different according to circumstances. I will 

 endeavour briefly to describe the process. If the carbons 

 had been already in use for some time, so that the small 

 quantities of gas set free from them by the high temperature 

 no longer produced any rapid change in the pressure, the 

 image of the arc of light was thrown by an assistant upon 

 the slit of the collimator of the telescope by the aid of an 

 achromatic lens, and the light from the upper or under carbon 

 was cut off from the slit by means of a suitable screen. If 

 the current used was weak, the spectrum of the carbon was 

 feeble, and showed at first no bright lines ; after a short time, 

 however, the hydrogen lines as well as a number of metallic 

 lines became sharply visible, first on the one carbon and then 

 on the other. The cause of these sudden changes is to be 

 sought, no doubt, in the fact that the arc does not burn 

 uniformly round both carbons, so that the parts of the carbon 

 points turned towards the collimator at times glow intensely, 

 at times are only feebly luminous. If the current is strong, 

 the spectrum of the carbons is a continuous one. 



It results from the observations described that hydrogen 

 is set free at both carbons ; we thus obtain a confirmation of 

 the conclusion that the carbons employed for electric lighting 

 always contain hydrocarbons, which the high temperature of 

 the arc decomposes, partially at least. The investigations of 

 Dewar * have shown that it is impossible to free carbon from 

 hydrogen compounds, even by lengthened heating in a current 

 of chlorine. We are altogether ignorant what composition 

 these compounds may have, or what changes they may suffer, 

 at the high temperature of the arc, except that there is no 

 doubt of the formation of acetylene in the arc in hydrogen, as 

 shown by Berthelot. But whether in this case we have really 



* Proc. Eoy. Soc. xxx. p. 87 (1880). 



