182 



Sir W. R. Grove on an Experiment on [Dec. 19, 



bands between the bright lines, and on attaching a small prism (with 

 which the spectroscope was provided) in front of the slit, so that the 

 separate spectra of the positive and negative lights could be juxta- 

 posed, I could trace several of the bands which appeared quite 

 distinctly in the positive spectrum, into the negative one ; but in the 

 latter they were very faint, while the converse case obtained with the 

 four bright lines I have mentioned, which were brilliant in the negative 

 spectrum and faint or normal in the positive. 



Although the four bright lines standing out in strong relief in the 

 negative spectrum was the more striking phenomenon to the eye, yet 

 the black band in the positive appearing in the space corresponding to 

 the bright yellow lig-ht in the negative spectrum is equally or possibly 

 more important. 



The positive light, far the brightest to the eye, is diffused into a 

 fluted spectrum of substantially equal intensity throughout, while the 

 negative dim light is concentrated into brilliant lines of intense lumi- 

 nosity. 



Another tube in which the vacuum was, I have no doubt, produced 

 by the absorption of carbonic acid by potash, and which may be 

 called a carbonic acid vacuum, gave a very different result from the 

 three I have mentioned. In it the light throughout was striated and 

 blue, or bluish, with a slig'ht purple tinge pervading the negative glow. 

 With this tube the spectra were strikingly different from those in the 

 air vacuo. There were in the negative spectrum of this tube six 

 bright lines, viz., extreme red, orange, greenish-yellow, green, greenish- 

 blue, and violet. The same lines with one exception were visible and 

 equally prominent throughout the whole of the tube. That exception, 

 which was noticed by Dr. Frankland, to whom I showed my experi- 

 ments, was the extreme red line which was apparent only in the 

 spectrum from the negative glow. 



On juxtaposing, by means of the prism, this spectrum with the 

 spectrum of the negative light in an air vacuum tube, one only of the 

 lines coincided, viz., the green line, the others were in entirely distinct 

 parts of the spectrum ; this was to be expected, as the one tube would 

 give mainly a nitrogen spectrum, the other a carbonic oxide one. 



I have long been convinced, and this is now, I believe, the prevalent 

 opinion, that the light of the electric discharge is an incandescence 

 of the intermedium through which it passes, and of the terminals 

 themselves (see " Correlation of Physical Forces," Gth edit., pp. 75, 

 et seq.). 



If this be so, then, the above experiments, i.e., those on the positive 

 and negative spectra in the same tube, must be either different spectra 

 of the same incandescent substances, or the attenuated gases must be 

 differently decomposed or united in the different parts of the tube, or 

 a different character of electric polarity must ensue in the positive 



