_Development of the Image._--Make a saturated solution of Gallic Acid in water by the directions given at page 261. Then dissolve forty grains of pure Nitrate of Silver in one ounce of distilled water. Pour into a flat porcelain dish a sufficient quantity of the Gallic Acid solution to flood the plate readily. Then measure it, and to each fluid ounce add _ten minims_ of the solution of Silver, or five minims in hot weather. It is important that no discoloration should occur on mixing these liquids together, to obviate which, observe the following precautions:--Clean the porcelain vessel very carefully with Nitric Acid or Cyanide before use. Employ a pure solution of Nitrate of Silver; and mix it with the Gallic Acid, in preference to adding the Gallic Acid to the Silver solution (read the remarks at p. 179). The picture may be expected to appear in five or ten minutes, and in one hour, or from that to four hours (p. 298), the development will be complete. It will not be necessary to keep the plates in motion, but simply to lay them side by side in the solution of the Gallic Acid. If in spite of all precautions the developer begins to blacken before the intensity has reached the proper point, it must be poured off and a fresh mixture prepared. This however will not often happen. Lastly, when a full amount of opacity has been obtained, wash the plate with water, and fix it in a solution of Hyposulphite of Soda, or dilute solution of Cyanide of Potassium. _Failures in the process._--Stains in the development may arise from using dirty dishes, or glasses which have been left in Gallo-Nitrate of Silver and improperly cleaned. It must be borne in mind that these impurities are not visible to the eye, although they produce the effect of discolouring the developer. A thorough cleansing with strong Nitric Acid or Potash will prove a remedy. Blisters, unless of large size, may often be disregarded, as they disappear on drying. General cloudiness may depend upon the film having been imperfectly washed. Irregular reduction at certain parts may be due to the Gelatine setting before the plate has become dry, or to stains produced by the finger applied to the upper edge of the plate.[54] [54] Since the above was written, Mr. Maxwell Lyte has communicated to the 'Photographic Journal' (vol. iii.) a dry process in which a _modified_ Gelatine is used. The change is produced by boiling a solution of gelatine with dilute Sulphuric Acid, which is afterwards neutralized and removed by means of chalk. The result is to destroy the gelatinizing property of the animal substance; the solution retains its fluidity on cooling, and the necessity of employing artificial heat in drying the plates is avoided. PART III. OUTLINES or GENERAL CHEMISTRY. OUTLINES or GENERAL CHEMISTRY. [Illustration] CHAPTER I. THE CHEMICAL ELEMENTS AND THEIR COMBINATIONS.