CHAPTER III.

ON THE DEVELOPMENT OF AN INVISIBLE IMAGE.

Simple experiments illustrating the process 25

Section I.--_Chemistry of the Substances, employed as Developers._--Development shown to be a process of reduction.--The chemistry of the principal reducing agents, Gallic Acid, Pyrogallic Acid, and the Protosalts of Iron 26

Section II.--_The Reduction of Salts of Silver by Developing Agents._--Reduction of Oxide of Silver--of Nitrate and Acetate of Silver.--Varied appearance of Metallic Silver when finely divided.--The reduction of the Iodide and other Salts of Silver containing no Oxygen 30

Section III.--_Hypothesis on the Formation and Development of the Latent Image._--Explanation of the terms under- and over-exposure.--Diagram of Molecular change.--Moser's experiments on development.--Curious perversions of development 34