1878.] On the Spectrum of the Electric Discharge. 181 



I. "Note of an Experiment on the Spectrum of the Electric 

 Discharge." By the Hon. Sir W. R. Grove, D.C.L., 

 V.P.R.S. Communicated December 19, 1878. 



The difference between the appearances at the positive and negative 

 terminal which an electric discharge presents in vacuum tubes has 

 struck many observers. The negative terminal is surrounded in what 

 is called an air vacuum with a blue glow extending to a considerable 

 distance from the platinum wire, and is generally bounded by a dark 

 space separating it from the crimson light of the positive wire ; it is 

 affected by the magnet, the light following the direction of the magnetic 

 curves, and a deposit of platinum on the glass tube appears in time in 

 the vicinity of the negative which is absent at the positive terminal. 

 I do not propose to enter more fully on these distinctions which have 

 been largely experimented on by M. Gassiot, Professor Pliicker, and to 

 some extent by myself. The recent announcement of Mr. Norman 

 Lockyer of observations on the spectra of bodies which were assumed 

 to be elementary, but which showed lines seeming to denote that they 

 were compound, led me to repeat some old experiments of mine on the 

 spectrum of the electric discharge, one result of which I have ventured 

 to communicate to the Royal Society. I had intended to mention 

 them in the discussion of Mr. Lockyer' s paper, but was not able 

 to be present at it. 



On November 24th last I examined, with a small spectroscope, by 

 Browning, the electric discharge in some Geissler's air vacuum tubes, 

 three of which I possessed. In these, which were of different shapes 

 and sizes, the effects were the same. The globes into which the 

 negative wire protruded were filled with a blue light more diffused as 

 it became more distant from the wire. The rest of the tube was filled 

 with a crimson light appearing to issue from the positive wire, and this 

 light was striated in the narrow parts of the tubes. 



The spectrum from what I will call the positive light presented a 

 series of numerous and variously coloured bands not greatly differing 

 in brightness, and showing what has been called the fluted or channelled 

 spectrum. The spectrum of the negative light was extremely different. 

 Four bright lines divided the spectrum, viz., yellow, green, blue, and 

 violet respectively, the distance between them increasing towards the 

 violet end. There was also a faint line at the extreme red, and the red 

 end of the spectrum was divided into two different tints, terminating 

 with the bright yellow line. In the positive spectrum there was a wide 

 black band, apparently an absorption band, overlapping the yellow 

 and a portion of the orange space. 



On looking for a longer time at the spectrum of the negative light, 

 my eye becoming more accustomed to it, I became able to detect other 



vol. xxviii. 



