in reply to his Animadversions. 423 



vapour, but the space occupied by the extra spectral red rays, 

 had undergone that change, which renders metals most suscep- 

 tible to the action of vapours, and a thick white line of vapour 

 very distinctly marked this calorific region, giving a spectral 

 image similar to the accompanying figure. It does not appear 

 that so long an exposure to solar influence was necessary, 

 for a similarly condensed spectrum was allowed to traverse 

 over a polished copper plate for a few hours. On submitting 

 this plate to vaporization, a line of thickly-deposited mercurial 

 vapour distinctly marked the path of the extra-spectral red 

 rays. I should explain that I mean the extreme red ray of 

 the spectrum, which is seen when it is looked at through a 

 cobalt-blue glass, and which has been made the subject of 

 much attention by Sir John Herschel, and some rays below 

 this extreme red ray. These rays cover a space which cor- 

 respond as nearly as possible with the large heat-spot in Sir 

 John HerschePs thermographic spectrum. I have been ex- 

 ceedingly anxious to repeat these experiments on other metals, 

 but the unfavourable state of the weather, and the advanced 

 season has compelled me to abandon this examination for the 

 present. 



M. Moser has stated that this "invisible light" will not 

 permeate glass. In my first communication on " Thermo- 

 graphy " (Phil. Mag. S. 3., vol. xxi. Dec. 1842, p. 464), para- 

 graph 9, I have shown that some influence is exerted through 

 red glasses which is not exerted through blue glasses. It may 

 be said that this was an influence radiated from the red glass, 

 in greater quantity than from the blue. I think, however, 

 that I have distinct proof of the permeation of the rays which 

 are active in producing these spectral images. 



Three very large flat white glass bottles were provided, and 

 filled with coloured fluids, blue, yellow and red. The light 

 had to pass through about 1^ inch of fluid in each case, this 

 was very carefully examined with the prism, and the depth of 

 colour adjusted until they represented, very fairly, the three 

 great divisions of the spectrum. Figures were cut in paper 

 and placed upon highly polished copper plates, being pressed 

 down by these bottles of coloured fluid. This arrangement 

 being left in the dark during a night, the plates were sub- 

 mitted to vapour, and they exhibited, each of them, impres- 

 sions of the paper figures, in which little or no difference could 

 be detected. The same arrangement was exposed for different 

 periods, varying from half an hour to two hours, to the influ- 

 ence of the sun's rays. Under the bottle containing the red 

 fluid, a most perfect image was formed by mercurial vapour, 

 even after the shortest exposure. Prolonged exposure gave a 



