Another point which has been kept in view, is to recommend, as far as possible, the employment of chemical agents which are used in medicine and vended by all druggists throughout the united kingdom. It is often an advantage to the Amateur to be able to purchase his materials near at hand; and, if the common impurities of the commercial articles are pointed out, and directions given for their removal, the 'London Pharmacopœia' will be found to include almost all the chemicals necessary for the practice of the Art. Great additions have been made to the Index of the present Edition, which is now so complete that a reference to it will at once point out the most important facts relating to each subject, and the different parts of the Work at which they are described. In conclusion, a hope is expressed that this 'Manual of Photographic Chemistry' may be found to be a complete and trustworthy guide on every point connected with the theory and practice of the Collodion process. _London, June 2nd, 1856._ PREFACE TO THE FOURTH EDITION. [Illustration] The Author has endeavoured to keep pace with the improvements which are daily being introduced in the science and art of Photography. In the present Edition alterations have been made in the style and general arrangement of the work, and additional matter has been inserted. Since the publication of the Third Edition, a series of experiments have been made on the manufacture of Collodion, the results of which have thrown further light upon the conditions affecting the sensitiveness of the excited film, and have enabled the writer to introduce an organic substance, "Glycyrrhizine," which will be found of service in making Photographic copies of Engravings and similar works of Art. Dr. Norris, of Birmingham, has within the last few months communicated a paper on _dry Collodion_, which places the theory of that subject upon a better footing than before. The Oxymel preservative process is now also thoroughly understood, and may be considered certain. In addition to the above, the "Albuminized Collodion" of M. Taupenot, which experience proves to be one of the best dry processes at present known, is included in this Edition. _King's College, London, April 6th, 1857._ ERRATA. Page 24, line 5, _for_ conditions _read_ condition. Page 115, line 32, _for_ Iodide _read_ Iodine. Page 194, line 15, _for_ p. 88 _read_ p. 188. [_Transcriber Note--Corrections have been applied_] CONTENTS. PART I. THE SCIENCE OF PHOTOGRAPHY. Page Introduction 1 CHAPTER I. Historical Sketch of Photography 6 CHAPTER II. THE SALTS OF SILVER EMPLOYED IN PHOTOGRAPHY. Section I.--_The Chemistry of the Salts of Silver._--The preparation and properties of the Nitrate of Silver--of the Chloride, Bromide, and Iodide of Silver.--The Chemistry of the Oxides of Silver 12