FORMULA No. 2.
| Pyrogallic Acid | 2 | grains. |
| Nitric Acid | 1 | drop. |
| Water | 1 | fluid ounce. |
FORMULA No. 3.
| Solution of Protonitrate of Iron | 1 | fluid ounce. |
| Alcohol | 20 | minims. |
In all these formulæ, if distilled water is not at hand, read the directions in the Vocabulary, Part III., Article "Water," for the best substitute.
Remarks upon these Formulæ.—Formula No. 1 is the most simple, since the solution can be used as a Bath, the same portion being employed many times successively. If it acts too rapidly, lessen the proportion of Sulphate of Iron. An addition of Nitric Acid, half a minim to the ounce, makes the image whiter and more metallic; but if too much is used, the development proceeds irregularly, and spangles of Silver are formed.
The Alcohol and Acetic Acid render the development uniform by causing the solution of Protosulphate to combine more readily with the film. The latter also has an effect in whitening the image and increasing its brightness.
Solution of Sulphate of Iron becomes red on keeping, from a gradual formation of persalt. When it is too weak, add more of the Protosulphate. The muddy deposit which settles to the bottom of the Bath is metallic Silver, reduced from the soluble Nitrate upon the plates.
Some operators add pure Nitrate of Potash to this developing solution, to form a small portion of Protonitrate of Iron. It is said to improve the colour slightly. The proportions are 10 grains of Nitrate of Potash to about 14 or 15 grains of Protosulphate of Iron.
Formula No. 2.—In this formula, if the colour of the image is not sufficiently white, try the effect of increasing the amount of Nitric Acid slightly. On the other hand, if the development is imperfect in parts, and patches of a green colour are seen, use three grains of Pyrogallic Acid to the ounce, with less Nitric Acid. A few drops of Nitrate of Silver solution added to the Pyrogallic, immediately before use, will augment the energy of development when blue and green spots occur.