Reference should also be made to a paper by Dr. Robinson, On the effects of Heat in lessening the Affinities of the Elements of Water, in the Transactions of the Royal Irish Academy, 1848, where he says that “when a platinum wire is traversed by a current gradually increased till it produces ignition, the first gleam that appears is not red, but of a colour which, when I first saw it, I compared to the ‘lavender ray’ discovered by Sir John Herschel beyond the violet, though I was surprised at seeing the tint of that most refrangible ray preceding the ray which is least so. It is quite conspicuous at about 865°; and as the mode in which it makes its appearance presents nothing abrupt or discontinuous, it seems likely that it is merely a transition from invisible rays excited at a lower temperature to ordinary light.”—p. 310.
[47] In the Bakerian Lecture for 1842, On the transparency of the Atmosphere, and the law of extinction of the solar rays in passing through it, by James D. Forbes, Esq., F.R.S., &c., will be found a most complete investigation of this subject.
The experiments were, for the most part, made in Switzerland with Sir John Herschel’s actinometer, and they prove satisfactorily,—“That the absorption of the solar rays by the strata of air to which we have immediate access, is considerable in amount for even moderate thicknesses.”
[48] After referring to several curious and instructive experiments, in which peculiar chemical changes are produced under the influence of the solar rays by their Heat, Sir John Herschel says:—
“These rays are distinguished from those of Light by being invisible; they are also distinguished from the pure calorific rays beyond the spectrum, by their possessing properties (of a peculiar character, referred to in former papers) either exclusively of the calorific rays, or in a much higher degree. They may perhaps not improperly be regarded as bearing the same relation to the calorific spectrum which the photographic rays do to the luminous ones. If the restriction to these rays of the term thermic, as distinct from calorific, be not (as I think, in fact, it is not) a sufficient distinction, I would propose the term parathermic rays to designate them. These are the rays which I conceive to be active in producing those singular molecular affections which determine the precipitation of vapours in the experiments of Messrs. Draper, Moser, and Hunt, and which will probably lead to important discoveries as to the intimate nature of those forces resident on the surfaces of bodies, to which M. Dutrochet has given the name of epipolic forces.”—On certain improvements in Photographic Processes, described in a former communication (Phil. Trans, vol. cxxxiii.); and On the Parathermic Rays of the Solar Spectrum, Phil. Trans, vol. cxxxiv.
The experiments of Mrs. Somerville, On the Action of the Rays of the Spectrum on Vegetable Juices (Phil. Transactions, vol. cxxxvii.), appear to connect themselves with this particular class of rays in a curious manner.
[49] Experiments on the influence of heat on differently-coloured bodies were first made by Dr. Hooke; and it was not until long after that Franklin made his ingenious experiments. Davy exposed to sunshine six equal pieces of copper, painted white, yellow, red, green, blue, and black, in such a manner that one side only was illuminated. To the dark side he attached a bit of cerate, ascertained by experiment to melt at 700. The cerate attached to the black became fluid first, the blue next, then the green and red, and lastly the yellow and white.—Beddoes’s Contributions to Physical Knowledge, and collected works of Sir Humphry Davy, vol. ii. p. 27.
[50] By reference to the Treatise on Heat, in the Encyclopædia Metropolitana, numerous suggestive experiments will be found, all bearing on this subject. Peschel’s Elements of Physics may also be consulted with advantage. The fact is, however, simply proved, as stated in the text, by placing the bulbs of delicate thermometers, so as to be completely involved in the petals of flowers exposed to sunshine, shading the upper portion of the stem of the instrument.
[51] Moser, On Vision, and on the Action of Light on Bodies: and also On Latent Light: Scientific Memoirs, vol. iii. Draper, On certain Spectral Appearances, and on the Discovery of Latent Light: Phil. Mag., Nov. 1842.
[52] A particular examination of this curious question will be found in the Author’s report On the Influence of the Solar Rays on the Growth of Plants: Reports of the British Association for 1847.