252 Dr. Gladstone on the Electric Light of Mercury. 



but when seen through cobalt glass it appeared reddish grey, or 

 nearly colourless. 



The prismatic analysis of this light explains at once the various 

 chromatic phenomena mentioned above. The brilliancy of the 

 yellow, blue, and violet rays accounts for the beautiful colour of 

 those objects which can reflect these rays ; while the feebleness 

 of the red shows the reason of the great changes which red sub- 

 stances invariably undergo — sometimes merely to brown, at other 

 times to purple, green, or whatever other colour in addition to red 

 is principally reflected by them. Thus the blood, wherever it 

 shows through the skin, as in the lips, appears of a bluish-purple 

 instead of a reddish hue. And this explains also why sulphate 

 of iron becomes perfectly colourless ; for the ordinary green tint 

 of this salt is due solely to its not transmitting with any freedom 

 the red rays : it transmits all others, and therefore all those 

 which prevail in the mercury-light. Hence it appears of the 

 same colour as the source of illumination itself, which the eye 

 ordinarily recognizes as white, though, as compared with the 

 sun, it is decidedly coloured. 



On searching afterwards for previous observations on the light 

 of mercury, I found that Prof. Wheatstone, in his brief notice 

 " On the Prismatic Decomposition of Electrical Light/' in the 

 Report of the British Association for 1835, mentions the spec- 

 trum of mercury as containing seven definite rays ; but he does 

 not further describe it. Masson*, in his elaborate paper, does not 

 mention the light from mercury; neither does Eobiquetf, nor 

 Secchi %. Angstrom §, however, gives a drawing of the lines that 

 make their appearance in such a light. They agree closely with 

 the more intense lines of my drawing, but with this important 

 difference — that he has not figured any violet ray except the least- 

 refrangible one. Thus he never saw the beautiful rays about 

 and beyond H. In his investigation, the Swedish philosopher 

 made the notable discovery that the spectrum of an electric spark 

 is ordinarily composed of two distinct spectra — the one belong- 

 ing to the gas across which the spark leaps, the other to the 

 conducting metal or other body. By observing different metals 

 in different gases, he was able to distinguish these two spectra ; 

 and he gives drawings of the lines due to oxygen, nitrogen, &c., 

 by a comparison of which with my map I find that the mercury 

 spectrum delineated by me is due solely to the gaseous metal, 

 and that no air was traversed by the electric spark in Professor 



* Ann. de Chim. et de Phys. 3rd series, vol. xxxi. p. 295. 

 t Comptes Rendus, October 31, 1859. 

 X Nuovo Cimento, vol. i. p. 405. 

 § Phil, Mag. S. 4. vol. ix. p. 327. 



