impression with it. When the hyposulphite of gold is used in gilding,
it requires less heat and a longer application, as there is some danger
of producing a glossy scum over some parts of the surface of the plate.
I prepare this salt as follows:

Dissolve one part chloride of gold and four parts
hyposulphite of soda in equal quantities of distilled water:
pour the gold into the hyposulphite solution, in the same
manner as in mixing the gilding solution; let it stand
until it becomes limpid; filter and evaporate to dryness.
Re-dissolve and add a few grains of burnt alum.

After standing a few hours, filter and evaporate again. If not
sufficiently pure, repeat the crystallization until it is so.
For gilding, dissolve in water and use in the same manner
as the common gilding solution.

N.B.--The four following mixtures were employed in Neipce's
process in his earliest experiments:

Aqueous Solution of Bichloride of Mercury.--Eight grains of bichloride
of mercury in 10,000 grains of distilled water.

Solution of Cyanide of Mercury.--A flask of distilled water is
saturated with cyanide of mercury, and a certain quantity is decanted,
which is diluted with an equal quantity of distilled water.

Acidulated White Oil of Petroleum.--This oil is acidulated by mixing
with it one tenth of pure nitric acid, leaving it for at least 48 hours,
occasionally agitating the flask. The oil, which is acidulated,