Photography is so essentially a chemical process that every photographer should have an interest in the chemicals he uses and in the reactions which they undergo.

In this and several articles which will follow, including such subjects as the chemistry of development, the chemistry of fixation, etc., no attempt will be made to give an account of advanced or even elementary chemical theory.

We will, however, give the reason for the use of symbols in designating the chemical elements, as these symbols are universally used by chemists.

All substances are made by the combination in various proportions of a limited number of elements of which about eighty exist. These elements combine together in different proportions to form bodies of fixed composition which are termed compounds.

Thus, one volume of the gaseous element hydrogen combines with one volume of the gaseous element chlorine to form two volumes of the compound hydrochloric acid gas.

This combination can be represented by what is called a chemical equation. Thus, if we write H for hydrogen, Cl for chlorine and H Cl for hydrochloric acid, we can represent the above combination by the equation H + Cl = HCl Hydrogen + Chlorine = Hydrochloric Acid.

It will be seen that an equation such as that given above is really a short-hand method of stating what happens, the elements which take part in the combination being designated by letters. These letters which stand for the elements are called the symbols of the elements, and by their combination the compounds of the elements are likewise indicated.

The art of photography is founded upon the fact that the compounds of silver, and especially its compounds with chlorine, bromine and iodine, are sensitive to light.

The earliest photographs were made by coating paper with silver chloride and using this to form images by its darkening under the action of light, but the sensitiveness of the silver chloride was too slight to use it in this way for forming images in the camera.

In order to get results which require less exposure to light, advantage is taken of the fact that it is not necessary for the light to do the whole work of forming the image. It is possible to expose the silver compound for only a short time to the light and then to continue the production of the image by chemical action, the process being termed "development."

Sensitive photographic materials therefore consist of paper, glass, or film coated with a sensitive layer which holds in suspension silver bromide or silver chloride. The sensitive layer which is coated on photographic material is called the emulsion. This emulsion consists of a suspension of the silver compound in a solution of gelatine. It is made by soaking gelatine in water until it is swollen and then dissolving it in warm water, gently warming and shaking the solution until all the gelatine is completely dissolved. The necessary bromide or chloride, e. g., potassium bromide or sodium chloride, is then added to the solution and dissolves in it. Meanwhile the right amount of silver nitrate to react with the amount of salts used has been weighed out and is dissolved in water. The silver nitrate solution is then added slowly to the solution of gelatine and salt and produces in it a precipitate of the silver compound, the mixing being done in the dark-room, since the silver compound produced is sensitive to light. If there were no gelatine in the solution the silver compound would settle down to the bottom and an emulsion would not be formed, but the gelatine prevents the settling and keeps the silver compound suspended evenly so that as the silver is added a little at a time the gelatine becomes full of the evenly precipitated silver distributed through the solution. If this solution is coated on a support such as paper or film and then cooled, the gelatine will set to a jelly, and when the jelly is dried we get a smooth coating of dry gelatine containing the sensitive silver compound suspended in it.

Materials which are to be used with development must contain no excess of soluble silver and the emulsion must be made so that there is always an excess of bromide or chloride in the solution, since any excess of soluble silver will produce a heavy deposit or fog over the whole of the surface as soon as the material is placed in the developer. In the case of Solio paper, however, which is not used for development but which is printed out, a chloride emulsion is made with an excess of silver nitrate, this having the property of darkening rapidly in the light, so that prints can be made on Solio paper without development, a visible image being printed which can be toned and fixed. Solio paper can be developed with certain precautions, but only by the use of acid developers or after treatment with bromide to remove the excess of silver nitrate.

In the early days of photography prints were usually made on printing-out papers, but at the present time most prints are made by the use of developing-out chloride or bromide papers, which are chemically of the same nature as the negative making materials and which are coated with emulsions containing no free silver nitrate.

Eastman Portrait Film Negative, Artura Print By W. O. Breckon Pittsburgh, Pa.

Eastman Portrait Film Negative, Artura Print By W. O. Breckon Pittsburgh, Pa.

Eastman Portrait Film Negative, Artura Print By W. O. Breckon Pittsburgh, Pa.

Eastman Portrait Film Negative, Artura Print By W. O. Breckon Pittsburgh, Pa.

Negative making materials, such as plates and films, always contain silver bromide with a small addition of silver iodide. The different degrees of sensitiveness are obtained by the temperature and the duration of heat which the emulsions undergo during manufacture, the more sensitive emulsions being heated at higher temperatures and for a longer time than the slower emulsions.