ic acid, water one hundred parts. This mixture should be allowed
to stand for several days; it improves by age.

"Use.--A few drops say, 6 to 12, of this mixture, should be put into
about 6 or 8 ounces of water; it will require frequent replenishing by
the addition of a few more drops. The plate should be coated over the
dry iodine to a red just bordering on a slate, and then exposed to the
mixture only sufficiently long to change the color. If this is not
done in less than six seconds it is not strong enough. Re-coat over
the iodine full one fourth as long as first coating."

This exceedingly volatile compound is difficult to control from its
instability; it is but little used. The impressions successfully
produced by this mixture are very brilliant, and possess a pleasing
peculiarity.

DRY SENSITIVES.

Hydrate of Lime.--The operation by which water is combined with lime is
called slaking. Take a piece of quick lime, common lime used in
mortar, and immerse it in warm water for about fifteen seconds; then
place it in an iron or tin vessel. It will soon begin to swell,
evolving a great deal of heat and emitting steam, and soon falls into a
fine powder, hydrate of lime. This should be well stirred and allowed
to cool, and then bottled in order to prevent it from giving off the
hydrate and recovering the carbonic acid from the atmosphere. The last
is detrimental to its use with bromine, and is one cause of the
complaint that "it will not take bromine." The hydrate of lime should,
not be dried over a heat, as has been supposed by many, for in that
case the hydrogen is expelled and it returns to a carbonate. It is
advisable to cool it in a damp place like a ground cellar. Much of the
lime in our market will not, except it be quite damp, combine with the
bromine. This is owing to impurities. Nothing is equal to
oyster-shell lime, which I use altogether.

Bromide of Lime.--In preparing large quantities of this, we adopt the
following method: Fill a four-quart bottle about two-thirds full of
hydrate of lime; pour into this about one or two ounces of bromine;
then shake well, add more of the bromine, shake well and let it stand
for a few hours, adding sufficient bromine to give it a fine red color.
It is better when kept in the large bottles, as it forms a more perfect
combination: in other words it improves by age.

Use.--Coat over the iodine to a rose red and then over this mixture to
a purple or slate;