the bottle
containing the iodine, and the yellowish-green gas disengaged will
readily combine with the iodine, forming a deep red liquid, and the
operation is complete. The use of chloride of iodine will be referred
to in connection with the Accelerators.

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Iodides.--The iodide treated with the oil of vitriol, instantly
produces a considerable deposit of iodine; and if the mixture be
heated, intense violent vapors are disengaged. The reaction is due to
the decomposition of oil of vitriol by iodohydric acid, water and
sulphurous acid being formed, and iodine set free. The iodides in
solution are decomposed by chlorine, iodine being precipitated, the
smallest quantity of which in solution is instantly detected by its
imparting to starch an intensely blue color.

Iodide of Potassium.*--This compound is easily made in the following
manner: Subject to a moderate heat a mixture of 100 parts of iodine, 75
of carbonate of potash, 30 of iron filings, and 120 parts of water.
This mass must be thoroughly dried and then heated to redness; the
resulting reddish powder is to be washed with water, and the solution
obtained filtered, and evaporated to dryness. It is found that 100
parts of iodine yield 135 parts of very white, but slightly alkaline,
iodide of potassium.

* I shall present the preparation of only a few iodides, and such as
are more intimately connected with the Daguerreotype.

Experiment.--On projecting dry pulverized iodide of potassium into
fused anhydrous phosphoric acid, a violent disengagement of iodine
takes place, attended by a transient ignition; fused hydrate of
phosphoric acid liberates iodine abundantly from iodide of potassium;
this reaction is accompanied by the phenomenon of flame and formation
of a considerable quantity of hydriodic acid.

Iodide of Mercury.--For the preparation of iodide of mercury, Dublanc
recommends to cover 100 grms. of mercury with 1 kilogrm. of alcohol,
to add 124 grms. of iodine gradually in portions of ten grms., and
agitating between each fresh addition, until the alcohol becomes
colorless again. After the addition of the last 4 grms. the alcohol
remains colored, the whole of the mercury having become converted into
iodide. The resulting preparation is washed with alcohol; it is
crystalline and of a hyacinth color.

Iodide of Silver.--This compound is formed upon every plate upon which
a Daguerreotype is produced. The vapor of iodine coming in contact
with t