The Honey Keeping Process.--The Oxymel Process.--Photography on dry Collodion.--Taupenot's Collodio-Albumen Process 288 PART III. OUTLINES OF GENERAL CHEMISTRY. CHAPTER I. THE CHEMICAL ELEMENTS AND THEIR COMBINATIONS. The more important Elementary Bodies, with their Symbols and Atomic Weights.--The Compounds formed by their union.--The class of Salts.--Illustrations of the nature of Chemical Affinity.--Chemical Nomenclature.--Symbolic Notation.--The Laws of Combination.--The Atomic Theory.--The Chemistry of Organic Bodies 305 CHAPTER II. Vocabulary of Photographic Chemicals 327 APPENDIX. Quantitative testing of Nitrate Baths.--Recovery of Silver from waste solutions.--Reduction of Chloride of Silver.--Mode of taking the Specific Gravity of liquids.--On Filtration.--The use of Test-papers.--The removal of Silver stains from the hands, etc.--Dr. Ure's tables of the strength of Sulphuric and Nitric Acid of different densities.--Table of Weights and Measures 371 A MANUAL OF PHOTOGRAPHIC CHEMISTRY. [Illustration] INTRODUCTION. In attempting to impart knowledge on any subject, it is not sufficient that the writer should himself be acquainted with that which he professes to teach. Even supposing such to be the case, yet much of the success of his effort must depend upon the manner in which the information is conveyed; for as, on the one hand, a system of extreme brevity always fails of its object, so, on the other, a mere compilation of facts imperfectly explained tends only to confuse the reader. A middle course between these extremes is perhaps the best to adopt; that is, to make selection of certain fundamental points, and to explain them with some minuteness, leaving others of less importance to be dealt with in a more summary manner, or to be altogether omitted. But independently of observations of this kind, which apply to educational instruction in general, it may be remarked, that there are sometimes difficulties of a more formidable description to be overcome. For instance, in treating of any science, such as that of Photography, which may be said to be comparatively new and unexplored, there is great danger of erroneously attributing effects to their wrong causes! Perhaps none but he who has himself worked in the laboratory can estimate this point in its proper light. In an experiment where the quantities of material acted upon are infinitesimally small, and the chemical changes involved of a most refined and subtle description, it is soon discovered that the slightest variation in the usual conditions will suffice to alter the result. Nevertheless Photography is truly _a science_, governed by fixed laws; and hence, as our knowledge increases, we may fairly hope that uncertainty will cease, and the same precision at length be attained as that with which chemical operations are usually performed.