M. TECHNOLOGY. 487 



because the warm water produces a swelling of the undis- 

 solved gelatine, and softens the lithographic ink. As a con- 

 sequence, when transferred to stone, which requires a certain 

 pressure, the softened portions are flattened out, and produce 

 blisters or irregular contours. 



CD 



This inconvenience is obviated by Paul's method, which 

 consists essentially in the substitution of albumen for gela- 

 tine, so that the washing can be done with cold water. The 

 paper is covered with a layer of albumen, combined with a 

 concentrated solution of the bichromate of potash. After a 

 sufficient exposure under the negative, the paper is covered 

 with lithographic ink, then immersed in cold water to dis- 

 solve the unaltered albumen, which is taken away with a soft 

 sponge. A very sharp picture is thus produced, which can 

 be transferred to stone. 3 7?, November 27, 1873, 553. 



PHOTO-ENGRAVING. 



Material progress is made in the application of photogra- 

 phy to various branches of engraving. The arts of photo- 

 lithography and photo-plate printing have lately been supple- 

 mented by certain processes which are called by those who 

 employ them " photo-engraving." These operations, though 

 their details are jealously kept secret, may be safely said to 

 depend largely upon producing, by means of bichromatized 

 gelatine, from an ordinary glass negative of the drawing or 

 engraving, a relief-plate in metal, either an electrotype or ste- 

 reotype, as may be desired. This may then be employed as 

 usual in the printing-press. Original pen drawings and cop- 

 ies from wood -cuts, steel or lithographic engravings, are re- 

 produced with exquisite fidelity, and, it is said, very cheaply. 

 The ability to employ a relief-plate in the printing-press, of 

 course, greatly lessens the expense of subsequent printing, as 

 compared with other methods. Photo-engraving of the char- 

 acter here referred to is at present carried on in New York 

 and Philadelphia. 



BLACK INK FOR MARKING BOXES, ETC. 



A cheap and beautiful article may be prepared by dissolv- 

 ing 10 parts of extract of logwood in 500 parts of water, and 

 adding 2 parts of yellow chromate of potash. It should be 

 stirred with a brush on using. 26 (7, 1874, vi., 56. 



