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