BROMIDE OF SILVER.--This salt, like the iodide, does not appear to be readily
changed by the action of light; but when combined with the nitrate of silver
it forms a very sensitive photographic preparation.

Paper prepared with this salt, blackens over its whole extent with nearly
equal intensity, when submitted to the prismatic spectrum. The most
characteristic peculiarity of the spectrum is its extravagant length.
Instead of terminating at the mean yellow ray, the darkened portion
extends down to the very extremity of the visible red rays.
In tint it is pretty uniformly of a grey-black over its whole extent,
except that a slight fringe of redness is perceptible at the least
refracted end. Beyond the red ray, an extended space is protected
from the agency of the dispersed light, and its whiteness maintained;
thus confirming the evidence of some chemical power in action,
over a space beyond the luminous spectrum, which corresponds with
the rays of the least refrangibility.

This salt is extensively used in photographic drawing.

PREPARATIONS OF GOLD.--Chloride of Gold, freed from an excess
of acid is slowly changed under the action of light;
a regularly increasing darkness taking place until it
becomes purple, the first action of the light being to whiten
the paper, which, if removed from the light at this stage,
will gradually darken and eventually develope the picture.
This process may be quickened by placing the paper in cold water.

Chloride of gold with nitrate of silver gives a precipitate of a yellow
brown color. Paper impregnated with the acetate of lead, when washed