PHOTO SALTS A name given first by Carey Lea to the coloured reduction product formed by the action of light on silver chloride, and later applied generally to the coloured salts obtained chemically by Lea, who considered the coloured reduction product to be c. chemical combination of silver chloride Aga and silver subchloride Hodgkin son, on the other hand, ascribed to it the formula as he considered that the ever present aqueous vapour or water played an important part in its formation. According to the latest theories, this product is a solid solution of in AgCI. Lea prepared the dark coloured products by the action of an alkaline hypochlorite or ferric chloride on finely divided metallic silver, by partial decomposition of silver oxide by heat and treatment with hydro chloric acid, by reducing silver chloride dissolved in ammonia by ferrous sulphate, or by the action of cuprous chloride on silver nitrate, etc. The colour of these photochlorides varied from purple to golden or rose red, and they were then capable of assuming the colours of the spectrum or any coloured objects to which they were exposed. These salts are of great interest, as no doubt they play an important part in the formation of the colours in the Seebeck or Poitevin pro cesses of heliochromy on paper. Photo-bromide
and photo-iodide have also been prepared, but they are not so colour-sensitive as the photo chloride.
The application of photo-mechanical repro duction processes to aluminium lithographic printing.
Photogravure printing in colours by inking the plate locally with tampons, masks being used to indicate the portion to be inked.
The term has also been applied ton litho graphic process of printing in colours from three etched stones.
Also a process of making coloured photographs on Whatman paper.
A modification of the photogravure process, worked out by Thos. Huson, R.I., and fully described in his book entitled " Photo Aquatint." The principal difference between this process and ordinary photogravure is that, instead of using different densities of etching solution, practically only one strength is used, and the negative, transparency, and carbon resist have accordingly to be adapted to the process.