Note 48 — Chapter VI

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.