Note 119 — Chapter VIII

“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.