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It will easily be understood that the bromide of iodine can be used with a greater or less quantity of water without altering the proportion which exists between the bromine and iodine."This article forms a very good dry accelerator, and is by some persons thought superior to all others, as it works with great uniformity, and is less liable to scum the plate in coating at high temperatures, or when the thermometer indicates a heat above 60 deg.

Bromide of Potassium--Is prepared by mixing bromine and a solution of pure potass together, and evaporating to dryness;it crystallizes in small cubes, and dissolves readily in water.

This agent is extensively employed in the paper and glass processes.

Bromide of Lime. This the principal accelerator used in the American practice, and is the best of all dry combinations at present employed.

There are many reasons why the dry is advantageous; these are too familiar to repeat.

"The bromide of lime may be produced by allowing bromine vapor to act upon hydrate of lime for some hours. The most convenient method of doing this is to place some of the hydrate at the bottom of the flask, and then put some bromine into a glass capsule supported a little above the lime.

As heat is developed during the combination, it is better to place the lower part of the flask in water at the temperature of about 50 deg.

Fah.; the lime gradually assumes a beautiful scarlet color, and acquires an appearance very similar to that of the red iodide of mercury.

The chloro-iodide of lime may be formed in the same manner; it has a deep brown color. Both these compounds, when the vapor arising from them is not too intense, have an odor analogous to that of bleaching powder, and quite distinguishable from chlorine, bromine, or iodine alone."Farther on, I have given, in connection with accelerators, a process I adopt, which is far less tedious and equally reliable.

Bromide of Silver--May be formed by pouring an alkaline bromide into a solution of nitrate of silver, in the shape of a white, slightly yellowish precipitate, which is insoluble in water and nitric acid, but readily dissolves in ammonia and the alkaline hyposulphites.

Chlorine easily decomposes bromide of silver, and transforms it into chloride.

M. Biot has expressed his opinion, that it is not possible to find any substance more sensitive to light than the bromide of silver.

This is true to a certain extent, but in combination with deoxidizing agents, other preparations have a decided superiority over the pure bromide of silver.

Bromide of Gold--Is readily prepared by adding a little bromide to the brown gold of the assayers, and allowing it to remain some time under water, or assisting its action by a gentle heat.

It forms a salt of a bright crimson color, but in its general properties is precisely similar to the chloride used in gilding.

Bromide of Magnesia--Is prepared in the same manner as bromide of lime.

This mixture is particularly adapted for hot climates, and is used in this country by some few who regard its use as a valuable secret.

Bromide of Starch.--This preparation is much used, but not alone.

It is combined with lime by putting about one part in measure of starch to four of lime. It is prepared by adding bromine to finely pulverized starch, in the same manner as bromide of lime.

(See Accelerators.)

Experiments with Bromine.--Place in a very flaring wine glass a few drops (say ten) of bromine, then place a small piece of phosphorus about one-twentieth of an inch in diameter.

Place the latter on the end of a stick from five to ten feet in length.

So place it that the phosphorus can be dropped into the glass, and in an instant combustion giving a loud report will be the result.

b. Expose a daguerreotype plate to the vapor of bromine, it assumes a leaden-grey color, which, blackens by light very readily.

Exposing this to mercury will not produce any decided action upon the lights.

Immerse it in the solution of the muriate of soda, and the parts unacted upon by light becomes a jet black, while the parts on which the light has acted will be dissolved off, leaving a clean coating of silver.

This will be a most decided black picture on a white ground.

c. Expose an impressioned plate, that has been sufficient time in the camera to become solarized, to the vapors of bromine, and the impression will be fully developed and exhibit no signs of solarization. The exposure over the bromine most be very brief.

Chlorine or iodine will produce the same result.

The latter is preferable.

Again, should the impressioned plate be exposed too long over the vapor of bromine, the impression would be rendered wholly insensitive to the mercurial vapor. Hence this method is resorted to for restoring the sensibility of the plate when there is reason to believe that the impression would not be a desirable one; as, for example, if a likeness of a child be wanted, and it had moved before the plate had been sufficiently long exposed in the camera, the plate may be restored to its original sensitiveness by re-coating over bromine, as above, thus saving the time and labor of re-preparing the plate for the chemicals.

d. If by accident (we would not advise a trial to any extent of this), you should inhale a quantity of the vapor of bromine, immediately inhale the vapor of aqua ammonia, as this neutralizes the dangerous effect of the bromine vapor.

every operator should be provided with a bottle of ammonia, as a little sprinkled about the chemical room soon disinfects it of all iodine or bromine vapor, and also tends to facilitate the operation in the camera.

IODINE.

History of Iodine.--This is one of the simple chemical bodies which was discovered in 1812 by M. Courtois, of Paris, a manufacturer of saltpetre, who found it in the mother-water of that salt.

Its properties were first studied into by M. Gay Lussac.

It partakes much of the nature of chlorine and bromine.