d in
warm water until the last washing water is quite neutral to test paper,
then drained and mixed with 200 parts of pure crystallized oxalic acid.
The mixture is then allowed to stand in the dark for several days at a
temperature not exceeding 30 deg. C. (86 deg. Fahr.) At first the
solution from green turns to a yellow green, and finally becomes almost
brown. At this moment the excess of ferric oxide is filtered out and the
liquor submitted to a quantitative analysis, the result of which leads to
ascertain the quantity of ferric oxalate in 100 parts of the solution and
the excess of oxalic acid. The solution should then be diluted with
distilled water, such as it contains 20 parts of ferric oxalate per 100
parts of water, and oxalic acid must be added in the proportion of from 6
to 8 per 100 of the ferric oxalate, taking into account the quantity of
acid the solution already contains. The solution should be kept in the
dark. It is altered by light.(20)

IRON CHLORATE SOLUTION
Ferric oxalate solution 100 parts
Potassium chlorate 0.4 parts

This solution is employed to obtain more contrasts.

PREPARATION OF THE SENSITIZING SOLUTION
Platinum solution 12 parts
Ferric oxalate solution 11 parts
Distilled water 2 parts

This solution gives very soft tones with intense black. To obtain more
brilliancy we use the following proportions:

Platinum solution 12 parts
Ferric oxalate solution 9 parts
Chlorate of iron solution 3 parts
Distilled water 2 parts

To obtain results comparable to those which the silver printing out
process gives, the following mixture is employed:

Platinum solution 12 parts
Ferric oxalate solution 8 parts
Chlorate of iron solution 4 parts
Distilled water 8 parts

For very weak negatives, reproductions of drawings, etc., we use--

Platinum solution 12 parts
Chlorate of iron 11 parts
Distilled water 2 parts(21)

To obtain proofs not completely black, as, for example, reproductions of
lead drawings, the solution may be diluted with half or the whole volume
of distilled water. But if the solution be applied on little absorbent
surfaces or on paper strongly sized it is not advisable to dilute it.

_Preparation, of the Paper.--_The paper should be kept slightly moist in
order that it does not too completely absorb the sensitizing solution.
Therefore, when the atmosphere is very dry,