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.