The present Chapter is divided into three Sections:--first, the chemical properties of the substances usually employed as developers;--second, their mode of action in reducing the Salts of Silver;--third, hypotheses on the action of light in impressing a latent image. SECTION I. _Chemistry of the various Substances employed as Developers._ Development is essentially a process of _reduction_, or, in other words, of _deoxidation_. If we take a certain metal, we can, by means of Nitric Acid, impart Oxygen to it, so that it becomes first an Oxide, and afterwards, by solution of the Oxide in the excess of acid, _a salt_. When this salt is formed, by a series of chemical operations the reverse of the former it may be deprived of all its Oxygen, and the metallic element again isolated. The degree of facility with which oxidation as well as reduction is performed, depends upon the affinity for Oxygen which the particular metal under treatment possesses. In this respect there is considerable difference, as may be shown by a reference to the two well-known metals, Iron and Gold. How speedily does the first become tarnished and covered with rust, whilst the other remains bright even in the fire! It is indeed possible, by a careful process, to form Oxide of Gold; but it retains its Oxygen so loosely that the mere application of heat is sufficient to drive it off, and leave the metal in a pure state. Silver, Gold, and Platinum all belong to the class of _noble_ metals, having the least affinity for Oxygen: hence their Oxides are unstable, and any body tending strongly to absorb Oxygen will reduce them to the metallic state. Observe, therefore, that the substances employed by the Photographer to assist the action of the light, and to develope the picture, act by removing Oxygen. The sensitive Salt of Silver is thus _reduced_, more or less completely, in the parts touched by light, and an opaque deposit results which forms the image.[7] [7] These remarks do not apply to the vapour of Mercury employed as a developing agent in the Daguerreotype. The chemistry of that process will be explained in a separate Chapter. The most important of the developers are as follows:-- Gallic Acid, Pyrogallic Acid, and the _Proto_salts of Iron. CHEMISTRY OF GALLIC AND PYROGALLIC ACIDS. a. _Of Gallic Acid._--Gallic Acid is obtained from _Gall Nuts_, which are peculiar excrescences formed upon the branches and shoots of the _Quercus infectoria_ by the puncture of a species of insect. The best kind is imported from Turkey, and sold in commerce as Aleppo Galls. Gall Nuts do not contain Gallic Acid ready formed, but an analogous chemical principle termed _Tannic Acid_, well known for its astringent properties and employment in the process of tanning raw hides.