varnish, and probably he employed oil of turpentine to wash away that bitumen which had not been rendered insoluble by the action of light. All btiumens and asphalts are more or less sensitive to light; light causing the material to become insoluble in the usual solvents ; and any bitumen or asphalt may be dissolved in benzol (from I of asphalt in 20 of benzol, to 1 of asphalt in 6 of benzol), and a metal plate having been coated as described in the preceding article, exposure is given under a negative if relief block is wanted, or under a positive if an intaglio plate is re-quired. A sufficient exposure having been given, the plate is developed as already described. If the plate is copper it may now be etched with a solution of ferric chloride (perchloride of iron), as explained under the head, Photogravure, and men-tioned in the article, Fish Glue Process. If the plate upon which the bitumen resist has been obtained is zinc, the per-chloride of iron may be used, but the method of etching described under Chemigraphy is more usual for zinc. An alternative method of working on zinc is to treat the zinc by the " litho-graphic " method, and print in the lithographic press.
Bitumen Process on Lithographic Stone. A lithographic stone, cleaned and prepared in the ordinary way, is coated with the sensitive bitumen varnish (the best being that prepared as directed in the preceding article), and after exposure under a negative, the soluble bitumen is removed as already directed (see preceding article). The stone, having been next washed by a vigorous stream of water, is dried, and treated by the usual lithographic method. A good handbook of lithography, which gives full details, is Richmond's " Grammar of Lithography," London, 1880, Wyman & Sons.
Half-tone or Process Blocks by the Bitumen Process. A copper or zinc plate is coated with bitumen, and it is exposed under a stippled negative of the kind mentioned in the article, Fish Glue Process. After development, or the removal of the superfluous bitumen, the plate is etched.
AlberinVs Reversal Bitume?i Process. Alberini finds that all the above methods may be reversed if crude bitumen is used, and if, after sufficient exposure, the film is developed by jlooding with alcohol, the action of the alcohol being assisted by gentle friction with a tuft of cotton wool. In this way the exposed por-tions of the film become disintegrated and removed, apparently