In the year 1853 an Englishman—Fox-Talbot, the gifted discoverer of calotype (_i.e._, the production of photographic negatives on paper by means of silver salts)—followed up the lines of these important observations, and discovered that the mixture of bichromate of potassium and glue entirely lost its solubility and power of swelling in cold water after exposure to light, and on this he based his process (heliography) of etching on metal plates. The plate, after receiving a coating of the chromated gelatine solution, was dried, exposed under a positive, and afterwards developed or washed out in warm water, all unexposed and consequently soluble portions being removed, while the exposed and therefore insoluble portions covered as a negative[A] the metal plate, and so protected it in those parts from the etching mordant employed, usually chloride of potassium or perchloride of iron. His successor in these researches was a Frenchman, Poitevin, who invested these processes with even greater practical importance. He, however, worked chiefly on paper with the bichromate mixed with albumen or gelatine, and attained results which were the first practical steps towards photo-lithography and carbon or pigment printing. Further on we shall return with more detail to these subjects. In the hands of Tessié du Motay and Marechal, of Metz, the discoveries of Poitevin derived further practical value, and to them we are indebted for the first advances in collotype, they in their practice using metal plates as a support. Joseph Albert, of Munich, followed, and introduced further and considerable modifications, amongst others utilising glass plates as a foundation. The want of adhesiveness of the chromated gelatine layer to its support was a source of great trouble to the earlier experimentalists, and was a matter giving rise to innumerable difficulties, which were, however, finally surmounted by the researches of Husnik, Albert, and Obernetter. [A] For the production of an intaglio plate the impression on the chromated gelatine must be printed from a positive, which will naturally give a negative on the copper plate. CHAPTER II. THE BICHROMATES IN CONJUNCTION WITH ORGANIC SUBSTANCES.