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. We are mainly indebted to Herschel's paper, published in 1840, for the knowledge we possess of gold as a photographic 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 induced. If, however, we remove the paper from the light, after an exposure of a few minutes, when a very faint impression, and oftentimes not any, is apparent, and hold it in the steam of boiling 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. If, while 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 of the hydriodate of potash, the oxidation is very rapidly brought on, and the darkness produced is much greater than that obtained by the other method ; but this plan is not often applicable. I have not yet been enabled to produce with the salts of gold any paper which should be sufficiently sensitive for use in the camera obscura.

Sir John Herschel devoted much attention to the examination of the salts of plantinum as well as gold. He found plantinum under nearly all circumstances very little sensitive to light, but the following were the results obtained with the salts of 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 purple in the sun. It passes also to the same purple hue in the dark, though much more slowly; so that, as a photographic combination, it is useless.

Paper impregnated with acetate of lead, when washed with perfectly neutral chloride of gold, acquires a brownish-yellow hue, and a sensibility to light, which, though not great, is attended with some peculiarities highly worthy of notice. The first impression of the solar rays seems rather to whiten than to darken the paper, by discharging the original colour, and substituting for it a pale grayish tint, which by slow degrees increases to a dark slate colour. But if arrested while yet not more than a moderate ash gray, and held in a current of steam, the colour of the part acted on by the sunshine, and that only, darkens immediately to a deep purple. The same effect is produced by immersing it in boiling water. If plunged in cold water, the same change comes on more slowly, and is not complete till the paper is dried by heat. A dry heat, however, does not operate this singular change.

If a neutral solution of the chloride of gold is mixed with an equal quantity of the solution of bichromate of potash, paper washed with this solution, and exposed to light, speedily changes, first to a deep brown, and ultimately to a bluish black. If an engraving is superposed, we have a negative copy, blue or brown, upon a yellow ground. If this photograph is placed in clean water, and allowed to remain in it for some hours, very singular changes take place. The yellow salt is all dissolved out, and those parts of the paper left beautifully white. All the dark portions become more decided in their character, and according as the solarization has been prolonged or otherwise, or the light has been more or less intense, we have either crimson, blue, brown, or deep black negative photographs.