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. [53] Ammianus Marcellinus ascribes the longevity and robust health of mountaineers to their exposure to the dews of night. Dew was employed by the alchemists in their experiments on the solution of gold. The ladies of old collected the “celestial wash,” which they imagined had the virtue of preserving their fine forms, by exposing heaps of wool to the influences of night radiation. It was supposed that the lean features of the grasshopper arose from that insect feeding entirely on dew: “Dumque thymo pascentur apes, dum rore cicadæ,” Virgil, Eclog. See some curious remarks by Boyle, _On the Power of Dew in Working on Solid Bodies_: Works of the Honourable R. Boyle, vol. v. p. 121. 1744. [54] See the _Researches on Heat_, by Professor James Forbes, in the Transactions of the Royal Society of Edinburgh; also Melloni’s papers on the same subject in the _Annales de Chimie_, several of which have been translated into the _Scientific Memoirs_, edited by Mr. Richard Taylor.