--------------------------+------------------------------+
0.2 grain per ounce | no increase after six hours. |
2.0 " " | about one-half an hour. |
20.0 " " | seven minutes. |
--------------------------+------------------------------+

I must here leave the _rationale_ of the process for the present, and
proceed with the next operation.

Our emulsion being cold, I add to it, for every 6 ounces of mixed
emulsion, 1 ounce of a saturated cold solution of potassium bichromate;
then, gently swirling the mixture round, a few drops of a dilute (1 to
8) solution of hydrochloric acid, and place it on one side for a minute
or two.

When hydrochloric acid is added to bichromate of potash, chromic acid is
liberated. Now, chromic acid has the property of precipitating gelatine,
so that what I hope to have done is to have precipitated the gelatine in
this emulsion, and which will carry down the silver bromide as well. You
see here I can pour off the supernatant liquid clear, leaving our silver
and gelatine as a clot at the bottom of the vessel.

Another action of chromic acid is, that it destroys the action of light
on silver bromide, so that up to this point operations can be carried on
in broad daylight.

The precipitated emulsion is now taken into the dark room and washed
until the wash water shows no trace of color; if there be a large
quantity, this is best done on a fine muslin filter; if a small
quantity, by decantation.