Sulphate of Iron, recrystallized 12 to 20 grains. Acetic Acid (glacial) 20 minims. Alcohol 10 minims. Water 1 fluid ounce. 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 _per_salt. 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. _Formula No. 3_, or Protonitrate of Iron, does not require any addition of Acid; but it will be advisable, in some cases, to add to it a few drops of Nitrate of Silver immediately before developing. It gives a bright metallic image, resembling that obtained by adding Nitric Acid to Protosulphate of Iron. The following process is commonly followed for preparing Protonitrate of Iron:--