[ 248 ■] 



XL. On the Luminosity of Gases through Electrical Dis- 

 charges. Supplement to the Paper on the Nature of Spectra. 

 By Eilhaed Wiedemann*. 



IN an investigation published in this Journalf I expressed 

 the opinion that by the electric spark, independently of 

 the temperature of a mixture of gases, certain particles are 

 rendered luminous, and that the luminosity is not a direct 

 consequence of a great rise of temperature, like the brightness 

 of incandescent solids, for instance, or that of sodium vapour 

 in a gas-flame. I have, on that account, compared the phe- 

 nomenon in question to the phenomena of fluorescence. 



New experiments have confirmed this view, and yielded the 

 result, that a gas may become luminous, on electricity passing 

 through it, while yet its temperature is far below 100°. 



For the experiments a discharge-tube was used consisting, 

 first, of a wider portion 30 millims. in diameter and 90 mil- 

 lims. in length, which was conically drawn out at its ends. 

 To one end a glass bulb provided with a glass cock was fused, 

 in the middle of which was an aluminium knob, serving as 

 electrode ; to the other end a capillary tube bent in the shape 

 of a U, diameter 0*854 millim., height of the U about 93 mil- 

 lims., was joined by fusion, to which was attached a glass bulb 

 with cock and aluminium electrode. 



The U-tube was placed in a calorimeter, which consisted of 

 a brass tube filled with oil of turpentine into which a ther- 

 mometer dipped, and was enclosed in a double-walled vessel 

 filled with water. The waterworth of the entire calorimeter 

 inclusive of the oil of turpentine, the thermometer, and the 

 immersed, portion of the U-tube, amounted in the experiments 

 to 8*846 grams. 



The discharge-tube was filled with air and exhausted (to 

 about 3 millims.) till the entire wide tube was completely 

 filled with continuous light when the discharges passed through 

 it. A very feeble stratification appeared momentarily in iso- 

 lated cases only. After the pressure was read off, the cock 

 communicating with the air-pump was closed. 



A Ruhmkorff induction-coil of medium size, with a mercury 

 interruptor, served as the source of electricity. By a simple 

 arrangement a black-writer was inserted, through a relais, in 

 a second current-circuit, which marked the number of the 

 closings of the Ruhmkorff, and therefore also the number of 

 the discharges through the Geissler tube. 



* Translated from a separate impression, communicated by the Author, 

 from Wiedemann's Annalen, vi. pp. 298-302. 

 t Phil. Mag. Feb. 1879, pp. 77-95. 



