PHOTO PRINTING. 379 



ordinary process of lithography the design to be printed is sketched 

 on a peculiar kind of stone with a greasy pencil, or with a pencil 

 containing a composition of the nature of soap, which will form a 

 free fatty acid on treatment with a mineral acid (Expt. 170) ; the 

 stone is then treated with dilute nitric acid or other acid which 

 will dissolve the stone in those parts where the surface is not pro- 

 tected by the greasy marks, just as in copper engraving (Expt. 272) ; 

 after washing with water the marks are finally obtained in slight 

 relief, and, being greasy, will take up printing ink or analogous 

 compositions with ease, whereas the other portions, not being 

 greasy but wet, will not take up the ink ; accordingly, on passing 

 an inking roller over the stone, ink is transferred to the marks, 

 and by then pressing paper on the inked stone the ink is printed 

 on to the paper, just as it is from the projecting portions of 

 the types cut into the form of letters and figures in ordinary 

 type printing. Now it happens that when a layer of chromatised 

 gelatin is altered in places by light it becomes more easily wetted 

 by printing ink and similar compositions and less readily wetted 

 by water than at the unaltered portions ; hence, by passing a 

 printing ink roller over the exposed plate and treating the inked 

 surface with water, the soluble parts of the gelatin are softened 

 and can be wiped or washed away, carrying the ink with it, whilst 

 the insoluble altered part remains with the ink adhering. The 

 plate can thus be printed from, just as an ordinary lithographic 

 stone. Various processes based on this property have been devised, 

 all belonging to the class of Collotypes. One modification now 

 largely employed, known as photolithography, consists in transfer- 

 ring the ink impression from the gelatin plate to a lithographic 

 stone, by printing from the plate on to the stone with lithographic 

 transfer ink ; the stone is then used in the ordinary way. Another 

 modification, termed photozincography, consists in coating a smooth 

 bright plate of zinc with chromatised gelatin, exposing behind a 

 negative, inking with lithographic ink, and dissolving away the 

 unaltered gelatin by water so as to remove the ink from the 

 unaffected parts, leaving it adhering to the altered gelatin. 

 Powdered resin is then dusted over the surface, which is gently 

 wiped ; the resin sticks to the greasy parts, but not elsewhere ; so 

 that on heating the resin melts and forms a protecting varnish or 

 "resist" (Expt. 272), preventing solution of the metal by acids 

 when the varnish is applied. The plate is then treated with acid, 

 and again resinised alternately several times until the metal is 

 sufficiently eaten away from the unprotected parts to form a picture 

 in relief capable of being printed from in the ordinary press. 

 A variety of other methods whereby photographs can be repro- 



