[117] See _Researches on Light_, by the Author.--Reference to any of the works of the alchemists will prove the prevalence of the idea expressed in the text. We find that gold was considered to be always under the influence of light and solar heat.--“It is said of gold that it waxeth cold towards daylight, insomuch that they who wear rings of it may perceive when the day is ready to dawn.”--_Speculum Mundi, or a Glass representing the face of the World_. Cambridge, 1643. [118] Daguerre’s Report to the Academy of Sciences: _La Daguerréotype Historique, et description des procédés du Daguerréotype et du Diorama_ (Paris, 1839); particularly the description of _Heliography_, by M. Niepce. See also the letters by Niepce, published for the first time in _Researches on Light_. [119] “If a solution of peroxalate of iron be kept in a dark place, or if it be exposed to 212° of Fahr. for several hours, it does not undergo any sensible change in its physical properties, nor does it exhibit any phenomenon which may be considered as the result of any elementary action. “If, however, it be exposed to the influence of solar light in a glass vessel provided with a tube, the concentrated solution of oxalate of iron soon presents a very interesting phenomenon: in a short time the solution receiving the solar rays, developes an infinite number of bubbles of gas, which rise in the liquor with increasing rapidity, and give the solution the appearance of a syrup undergoing strong fermentation. This ebullition always becomes stronger, and almost tumultuous, when an unpolished glass tube is immersed in it with a small piece of wood; the liquid itself is afterwards thrown into ascending and descending currents, becomes gradually yellowish, turbid, and eventually precipitates protoxalate of iron, in the form of small brilliant crystals of a lemon-yellow colour, gas continuing to evolve.” _Chemical action of light, and formation of Humboldtine by it_; Phil. Mag., 1832, second series.--“When a solution of platinum in nitro-muriatic acid, in which the excess of acid has been neutralized by the addition of lime, and which has been well cleared by filtration, is mixed with lime-water in the dark, no precipitation to any considerable extent takes place for a long while,--indeed, none whatever, though after very long standing a slight flocky sediment is formed, after which the action is arrested entirely. But if the mixture, either freshly made or when cleared by subsidence of this sediment, is exposed to sunshine, it instantly becomes milky, and a copious formation of a white precipitate (or a pale yellow one, if the platinic solution be in excess) takes place, which subsides quickly and is easily collected. The same takes place more slowly in cloudy daylight.”--_On the action of light in determining the precipitation of Muriate of Platinum by Lime water_; being an extract from a letter from Sir John F. W. Herschel, K.H., F.R.S., &c., to Dr. Daubeny.--Phil. Mag. 1832.