Section I.--_Collodion._--Pyroxyline--its varieties--its chemical composition.--Means of obtaining Nitro-Sulphuric Acid of the proper strength.--Solvents for Pyroxyline.--Variation of physical properties in different samples of Collodion.--The changes which Iodized Collodion undergoes by keeping 75 Section II.--_The Chemistry of the Nitrate Bath._--Its solvent action on Iodide of Silver.--Acidity and Alkalinity of the Nitrate Bath.--Formation of Acetate of Silver in the Bath.--The substances which decompose the Bath.--Changes in the Nitrate Bath by use 86 Section III.--_The Conditions which influence the Formation and Development of the Latent Image._--Causes which increase or diminish the sensitiveness of the film to Light.--Conditions which hasten or retard development 91 Section IV.--_On certain irregularities in the Developing Process._--Effect of particular states of the Bath, and of the Collodion, in producing clouding of the Image, and of acids in obviating it 103 CHAPTER VII. ON POSITIVE AND NEGATIVE COLLODION PHOTOGRAPHS. Definition of the terms Positive and Negative.--The same Photograph often capable of representing both varieties 106 Section I.--_On Collodion Positives._--The Collodion and Nitrate Bath best suited for Positives.--Peculiarities of Pyrogallic Acid, and the Protosalts of Iron employed to develope Collodion Positives.--The colour of Positives affected by the length of exposure to light.--A Process for whitening Glass Positives by means of Bichloride of Mercury 108 Section II.--_On Collodion Negatives._--The Collodion and Nitrate Bath best adapted for Negatives.--Use of Glycyrrhizine to increase the intensity.--Developing solutions for Negatives.--Conversion of finished Positives into Negatives 113 CHAPTER VIII. ON THE THEORY OF POSITIVE PRINTING. Section I.--_The Preparation of the Sensitive Paper._--Its darkening by light.--The conditions which affect its sensitiveness and the vigour of the Image.--The colour of the print influenced by the preparation of the paper 121 Section II.--_The Processes for Fixing and Toning the Proof._--Conditions of a proper fixing.--The Salts of Gold used as toning agents.--The properties of the single fixing and toning Bath, with the conditions which affect its action 128 Section III.--_The Author's Photographic Researches._--The chemical composition of the Photographic Image.--The various agencies destructive to Photographs.--The action of damp air upon Positive Prints.--The change in properties of the fixing Bath by constant use 140 Section IV.--_The Fading of Photographic Prints._--The causes which produce it.--The comparative permanency of Prints.--The mode of testing permanency 160 CHAPTER IX. ON THE DAGUERREOTYPE AND TALBOTYPE PROCESSES.