he silver surface, forms an iodide which is peculiarly sensitive
to light.

The various colors produced are owing to the thickness of the coating,
and the maximum sensibility of the coating, as generally adopted, is
when it assumes a deep yellow, or slightly tinged with rose color.

This compound is largely employed in most photographic processes on
paper, and may be easily prepared by the following formula: By adding
iodide of potassium to a solution of nitrate of silver, a
yellowish-white precipitate of iodide of silver is obtained, which is
insoluble in water, slightly soluble in nitric acid, and soluble in a
small degree in ammonia, which properties seem easily to distinguish it
from the chloride and bromide of silver. Chlorine decomposes it and
sets the iodine free, and chlorohydric acid converts it into a
chloride. It fuses below a red heat. Although the effect of light on
the iodide is less rapid than on the chloride, the former sooner
turning black, assuming a brown tinge; but when in connection with
gallic acid and the ferrocyanate of potash, it forms two of the most
sensitive processes on paper.

Iodide of silver dissolves easily in a solution of iodide of potassium,
and the liquid deposits in evaporation crystals of a double iodide.

Iodide of Gold.--If a solution of potassium be added to a solution of
chloride of gold, a precipitate of iodide of gold takes place, soluble
in an excess of the precipitate. A little free potash should be added
to combine with any iodide that may chance to be set free by the
chloride of gold.

Iodide of Lime is prepared by adding iodine to hydrate of lime (which
will be referred to farther on) until the mixture assumes a light
yellow shade, when wanted for combinations with accelerators, or to a
dark brown when employed for the first coating. This latter mixture
has been sold in our market under the name of "Iodide of Brome."

Iodide of Bromine.--(See page 76.)

Experiments with Iodine.--Place a plate which has been exposed in the
camera over the vapor of iodine for a very brief period, and it will
present the appearance of the impression having been solarized.

b. Upon a Daguerreotype plate, from which an impression has been
effaced by rubbing or otherwise, the picture may be made to reappear by
merely coating it over with iodine.

c. Place in a vessel a little water, into which put the smallest
possible quantity of free iodine and add a little starch, and the