The intention of the author in writing this work, is to impart a thorough knowledge of what may be termed the "First Principles of Photography," that the amateur may arm himself with a theoretical acquaintance with the subject before proceeding to the practice of it. To assist this object, care will be taken to avoid needless complexity in the formulæ, and all ingredients will be omitted which are not proved to be of service. The impurities of chemicals will be pointed out as far as possible, and special directions given for their removal. Amongst the variety of Photographic processes devised, those only will be selected which are correct on theoretical grounds, and are found in practice to succeed. As the work is addressed to one supposed to be unacquainted both with Chemistry and Photography, pains will be taken to avoid the employment of all technical terms of which an explanation has not previously been given. A SKETCH OF THE MAIN DIVISIONS TO BE ADOPTED, WITH THE PRINCIPAL SUBJECT-MATTER OF EACH. The title given to the Work is "A Manual of Photographic Chemistry," and it is proposed to include in it a familiar explanation of the nature of the various chemical agents employed in the Art of Photography, with the rationale of the manner in which they are thought to act. The division adopted is threefold:-- Part I. enters minutely into the _theory_ of Photographic processes; Part II. treats of the _practice_ of Photography upon Collodion; Part III. embraces a simple statement of the main laws of Chemistry, with the principal properties of the various substances, elementary or compound, which are employed by Photographers. Part I., or "the Science of Photography," includes a full description of the chemical action of Light upon the Salts of Silver, with its application to artistic purposes; all mention of manipulatory details, and of quantities of ingredients, being, as a rule, omitted. In this division of the Work will be found nine Chapters, the contents of which are as follows:-- Chapter I. is a sketch of the history of Photography, intended to convey a general notion of the origin and progress of the Art, without dwelling on minute particulars. Chapter II. describes the Chemistry of the Salts of Silver employed by Photographers; their preparation and properties; the phenomena of the action of Light upon them, with experiments illustrating it. Chapter III. leads us on to the formation of _an invisible image_ upon a sensitive surface, with the development or bringing out to view of the same by means of chemical re-agents. This point, being of elementary importance, is described carefully;--the reduction of metallic oxides, the properties of the bodies employed to reduce, and the hypotheses which have been entertained on the nature of the Light's action, are all minutely explained. Chapter IV. treats of the fixing of Photographic impressions, in order to render them indestructible by diffused light.