rise to the formation of the metallic
ferrocyanide, of which the image consists, are quite complicated. Under
the luminous agency the uranic nitrate is first reduced, then the uranous
oxide acts on the cupric nitrate, forming cupric oxide, which is finally
reduced to the metallic state. This metal now converts the ferricyanate
in the ferro compound, which, by another action, forms both cupric and
uranic ferrocyanate.

The following uranium process gives black impressions:

In a saturated solution of tartaric acid dissolve freshly precipitated
ferric oxide, and keep the solution--ferric tartrate--in the dark. To
prepare the sensitizing solution, dissolve 20 parts of uranic nitrate and
from 1 to 3 parts of tartaric acid in 100 parts of water, and add a small
quantity of ferric tartrate, the proportion varying with the tint desired:
an excess gives a blue black. With this solution brush the paper over,
and, when dry, expose under the negative cliche, then develop with a
solution of potassium ferricyanate at 4 per 100 of water. To fix, it
suffices to wash in water, renewed three or four times.

As pointed out by Mr. B. J. Burnett (see Introduction), many photographic
processes can be devised by basing them upon the various chemical changes,
of which uranous oxide, reduced by light from the uranic nitrate or
sulphate, is susceptible by means of metallic or organic reagents.

In the Appendix some of the most important processes, with or without
silver salts as reagents, will be described.

THE ANILINE PROCESS.

The aniline process was published in 1865, by Mr. Willis, the inventor of
the platinotype.(11) It is based on the oxidation of aniline by chromic
acid, thus: A sheet of paper brushed with a solution of potassium
bichromate and sulphuric acid, dried, and after insolation under a cliche
exposed to the fumes of aniline which, in reacting with the chromic
compound not reduced by light, forms a blue-black image. _The process
gives, consequently, a positive impression from a positive cliche._

There are various methods of operating; we will briefly describe them.

SENSITISING SOLUTION.
1. Potassium 6 parts
bichromate
Sulphuric acid 6 parts
Magnesium chloride 10 parts
Water 150 parts

Willis recommended 10 parts of solid phosphoric acid instead of sulphuric
acid; the latter forms a preparation about twice more rapidly redu