and because it assists in fixing the layer of gold, rendering it
less susceptible of being removed in scales when heated too much."

Rotten Stone.--"Purchase the best ground rotten stone
of the druggist, put a few ounces at a time in a wedgewood
or porcelain mortar, with plenty of clean rain water.
This should have about forty drops of nitric acid to the quart.
Grind well, and after letting the mortar stand two minutes,
pour into a third. After remaining undisturbed eight minutes,
finally pour off into a fourth to settle. Rinse back the sediment
in the second and third, and grind over with a new batch.
Repeat the operation till you have all in the fourth vessel.
Let this stand several hours, and pour off the water very carefully.
Set the deposit in the sun, or by a stove to dry.
When perfectly dry, pulverize, and it is ready for use.
With a little trouble you will obtain in this way a much
better article than can generally be bought of dealers.
For the last washing, alcohol, or a mixture of alcohol
and water, is preferable."

Potassa Solution.--The use of a solution of potassa in the preparation
of the plate was suggested in the early history of the Daguerreotype.
It was thought to possess some peculiar property for improving the tone
of the impression. It is used for moistening the rotten stone in polishing
the plate, and may be prepared by putting about an ounce and a half
of alcohol in a close bottle, and add half a stick of caustic potash.
This will soon become of a deep red color. For use, fill your small bottle,
having a quill in the cork, with alcohol, and add a few drops of the above,
or enough to change it to a bright orange or saffron color.