even
at ordinary temperature, to place the receiver in an ice bath. For
manner of using, see farther on, under head of Accelerators.

Bromides.--A bromide treated with oil of vitriol, disengages
chlorohyadic acid; but vapors of bromine are constantly disengaged, at
the same time imparting a brown color to the gas. If the bromide be
treated with a mixture of sulphuric acid, and peroxide of manganese,
bromide is only disengaged. A solution of a bromide gives, with of
nitrate {76} silver, a light yellowish white precipitate of bromide of
silver, which is insoluble in an excess of acid, and readily dissolves
in ammonia. The precipitated bromide is colored by light like the
chloride, but is immediately tinged brown, while the chloride assumes
at first a violet hue. The bromides, in solution, are readily
decomposed and chloride being set free, colors the liquid brown.

In the whole range of heliographic chemicals there is probably not
another collection less understood and being so productive of
interesting investigation as the bromides.

Bromide of Iodine.--M. de Valicours furnishes us with the best method
for preparing this mixture:

"Into a bottle of the capacity of about two ounces, pour thirty or
forty drops of bromine, the precise quantity not being of importance.
Then add, grain by grain, as much iodine as the bromine will dissolve
till quite saturated. This point is ascertained when some grains of
the iodine remain undissolved. They may remain in the bottle, as they
will not interfere with the success of the preparation.

"The bromide of iodine thus prepared, from its occupying so small a
space, can very easily be carried, but in this state it is much too
concentrated to be used. When it is to be employed, pour a small
quantity, say fifteen drops, by means of a dropping-tube, into a bottle
containing about half an ounce of filtered river water. 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 dryn