A Manual Of Photography

SALTS OF GOLD AS PHOTOGRAPHIC AGENTS. JO are of different degrees of sensibility, but for surface drawings they may be used ; and in copying of leaves or flowers, beautiful pictures, which appear to glow with the natural colours, ai procured. Section IX.—Salts of Gold as Photographic Agents. It is well known that gold is revived from its ethereal solution by the action of light, and that the same effect takes place when the nitro-muriate of gold is spread on charcoal AV e are mainly indebted to Herschel’s paper, published in 1840, for the know- ledge we possess of gold as a photographie agent. Considering it probable that the required unstable equilibrium might be induced in some of the salts of gold, I was induced to pursue a great many experiments on this point. In some cases where the paper was impregnated with a mordant salt, the salt of gold was darkened rapidly, without the assistance of light, in others, the effect of light was very slow and uncertain. By washing paper with muriate of barytes, and then with a solution of the chloride of gold, a paper, having a slight pinky tint is procured; by exposing this paper to sunshine it is at first whitened, and then, but very slowly, a darkening action is in- duced If however, we remove the paper from the light, attei an exposure of a few minutes, when a very faint impression and oftentimes not any, is apparent, and hold it in the steam of boil- ing water, or immerse it in cold water, all the parts which were exposed to the light are rapidly darkened to a full purple brown, leaving the covered portions on which the light has not acted, a pure white, producing thus a fine negative drawing. ,w nt such a paper, or any other paper prepared with the chloride of gold is exposed to the sun, we wash it with a weak solution 01 the hydriodate of potash, the oxidation is very rapidly brought on and the darkness produced is much greater than that ob- tained by the other method ; but this plan is not often applicable. I have not yet been enabled to produce with the salts 01 gold any paper which should be sufficiently sensitive for use m the Sir John Herschel devoted much attention to the examination of the salts of platinum as well as gold. He found platinum under nearly all circumstances very little sensitive to light but the following were the results obtained with the salts ot gold. If paper impregnated with oxalate of ammonia be washed with chloride of gold, it becomes, if certain proportions be hit, pretty sensitive to light ; passing rather rapidly to a violet purp cm