there is great tendency to frill--such, for instance, as when a soft
sample of gelatine has been employed, or old decomposed emulsion worked
in with the fresh emulsion--it will in such cases be safer to put the
plates in the normal-bath for a few minutes previous to immersing them
in the acid bath.

Potash alum is obtained tolerably pure in commerce in colorless
transparent crystalline masses, having an acid, sweetish, astringent
taste. It is soluble in 18 parts of water at 60 deg. F., and in its own
weight of water at 212 deg. F.; but the excess crystallizes out upon
cooling. The solution reddens litmus paper, and, when impure, usually
contains traces of oxide of iron. Upon the addition of either caustic
soda or potash, a white gelatinous precipitate is formed (hydrate of
alumina), which is soluble in excess of the reagent employed. The
precipitate thus obtained has much of the character of the opalescent
film sometimes observed on gelatine plates, when dry, which have been
soaked in alum, and not well washed afterward.

Alkaline carbonates--such as washing soda, for instance--precipitate
hydrate of alumina, which does not dissolve in an excess of the
reagents, and carbon dioxide is evolved.

Ammonia hydrate produces a precipitate in a much finer state of divison,
which does not dissolve in excess when examined in a test-tube, it
somewhat resembles thin starch paste.

The presence of traces of iron may be known by adding a few drops of
hydrochloric acid to a small quantity of a saturated solution of alum
in a test-tube, to which add strong liquid ammonia; should any iron be
present, the mixture will have a reddish-brown tinge when examined over