A Manual Of Photography

10 HISTORY 0F PHOTOGRAPHY. the Solfaterra under the form of octahedral crystals, known under the name of ruby of arsenic, effloresces by the light;” and that ordinary native realgar from Japan changes to orpi- ment by exposure to sunshine. In 1806, 1 ogel exposed fat, carefully protected from the influence of the air, to light, and found that it became in a short time of a yellow colour, and acquired a high degree of rancidity. Vogel subsequently discovered that phosphorus and ammonia exposed to the sun's rays were rapidly converted into phosphu- retted hydrogen, and a black powder, phosphuret of ammonia. Me also noticed that the red rays produced no change on a solution of corrosive sublimate (bichloride of mercury) in ether, but that the blue rays rapidly decomposed it. Dr. Davy, much ]nore recently, repeated a similar set of experiments to those of Vogel. Me found that corrosive sublimate was not changed by exposure; but that the Liquor Hydrarg. Oxymur, of the old London Pharmacopoeia quickly underwent decomposition in the sunshine, depositing calomel (chloride of mercury.) Seebeck, in, and subsequently to 1810, made some important additions to our knowledge of the influences of the solar radiations, the most striking of his statements being the pro- duction q/ colour on chloride of silver ; the violet rays rendering it brown, the blue producing a shade of blue, the yellow pre- serving it white, and the red constantly giving a red colour to that salt. Sir Henry Englefield, about the same time, was enabled to show that the phosphorescence of Canton’s phos- phorus was greatly exalted by the blue rays. Gay-Lussac and Thénard, being engaged in some investigations on chlorine, on which elementary body Davy was at the same time experimenting, observed that hydrogen and chlorine did not combine in the dark, but that they combined with great rapidity, and often with explosion, in the sunshine, and slowly in diffused light. Seebeck collected chlorine over hot water, and, combining it with hydrogen, placed different portions of it in a yellowish-red bell glass and in a blue one. In the blue glass combination took place immediately the mixture was exposed to daylight ; but without explosion. The mixture in the red glass was exposed for twenty minutes without any change ; but it was found that the chlorine had undergone some alteration, probably a similar one to that subsequently noticed Dy Br. Draper, who found that chlorine having been exposed to sunshine would unite with hydrogen in the dark. If the gases were placed in a white glass and exposed to sunshine, they exploded; but if the gas had been previously exposed to the action of the solar radiations in the yellow-red glass, it