CHAP, vni.] HELIOGRAPHY- PHOTOLITHOGRAPHY. 319 



lead) to the action of a hydraulic press. The reliefs of the gelatine 

 sink into the metal 



The metallic impression thus obtained is used in a printing process 

 which is absolutely original. It consists in pouring an inky fluid 

 (gelatine coloured with carbon, or otherwise) on the levelled metal 

 plate and superposing a sheet of resinized paper and submitting ifc to 

 pressure in a heavy press. What is the result ? The sheet of paper, 

 pressed by a plate of glass, drives the excess of ink to the edges of 

 the mould, and the hollows alone remain filled. As soon as the 

 gelatine is set, the paper, taken out of the press, carries with it the 

 coloured gelatinous coating. The latter then forms on the paper a 

 drawing in relief, which, however, diminishes as the gelatine dries. 

 Wherever the gelatine is thickest, the tint is strongest, shading off to 

 white where there is no relief. 



It is impossible to give in detail an idea of the numerous processes 

 of heliographic printing. They are all based on the fact that chro- 

 mated gelatine when exposed to light becomes non-absorbent of water 

 in exact proportion to the intensity with which, and length of time 

 for which, the luminous rays act. Tf a gelatine film, supported on 

 glass per se, be exposed under a negative possessing lights and shades 

 it will absorb water according to the density of different parts of the 

 negative. When a soft lithographic roller coated with greasy ink is 

 passed over it while moist, the ink will adhere in proportion to the 

 non-absorption of the water. A piece of paper placed over such an 

 inked-in surface and pressed into it in a printing or lithographic press 

 will take away an impression, giving the lights and shades in proper 

 gradation. 



It is only fair to mention the names of some of the inventors 

 Baldus, Nigre, Placet, Albert of Munich, Edwards, Du Gardin, Tessie 

 du Motay, Waterhouse, Jeanrenaud and Thiel. 



The results are certainly remarkable, but many of the processes are 

 defective in one particular, viz., the difficulty of printing off a great 

 number of impressions from the same surface. While this diffi- 

 culty remains heliography will be incomplete ; it will be unable to 

 respond to the wants of artistic industry, and of the trade, which 

 require a low price, and which is impossible while the printing-off 

 remains circumscribed. With some more modern processes, such as 

 those of Edwards and Thiel, the defect does not exist, it being possible 



