NATURAL PHILOSOPHY. 229 



soft brush, a coating composed of animal soap, gelatine, and alum, upon all 

 its faces ; when the coating is dry, the surface which is to receive the picture 

 is placed for some minutes in a solution of muriate of ammonia (sal-ammo- 

 niac), then dried; then on a bath of nitrate of silver of t\venty per cent., 

 and then dried. A cliche upon glass or paper is then applied on the 

 wood by means of a peculiar frame, permitting the process of the reproduc- 

 tion to be watched. When satisfactory, the picture is fixed by means of a 

 saturated bath of hyposulphite of soda. A few minutes is enough ; it is 

 then washed for only five minutes. The first coating preserves the wood 

 from moisture ; and eight months of experience have proved to the inventor 

 that the employment of alum and a hyposulphite, in place of destroying the 

 wood, gives it a great strength favorable to the engraving. Comptes Rendus, 

 October, J857. 



Another process, recently patented by E,. Rice, of Worcester, Mass., and 

 which is now practically applied, consists in preparing the wooden blocks 

 first of all with a thin solution of asphaltum or bitumen, ether and lamp- 

 black, rubbed into the pores of the wood. This ethereal solution of asphalt 

 is put on the surface of the block with a rag, brush or sponge, and then some 

 fine lampblack is also rubbed in dry ; the surface of the block is afterwards 

 polished on a cushion, when it acquires a smooth, jet black, glossy appear- 

 ance. After this, it is treated by the common photographic process ; 

 namely, coated with collodion rendered sensitive by nitrate of silver, then 

 put into the camera, the picture taken, then fixed and dried in the usual 

 manner. 



OX PHOTO-GALYAXOGRAPHY, OR ENGRAVING BY LIGHT AXD 



ELECTRICITY. 



The following is a detailed description, copied from the specification of 

 the patent, of the very remarkable method of engraving, by the combined 

 process of photography and electricity, invented by M. Pretsch, late manager 

 of the imperial printing office at Vienna. A brief notice of this invention 

 was given in the Annual of Scientific Discovciy for 1857, page 211. 



My invention, says M. Pretsch, consists in adapting the photographic 

 process to the purpose of obtaining a raised or sunk design, on a glass or 

 other suitable plate covered with glutinous substances, mixed with photo- 

 graphic materials, which design can then be copied by the electrotype pro- 

 cess, so as to procure plates suitable for printing purposes. The operator 

 first coats a glass plate with a gelatinous or glutinous solution, suitably pre- 

 pared with chemical ingredients, sensitive to light, as follows : One part 

 of clear glue is soaked in about ten parts of distilled water, but the quantity 

 of water depends upon the strength of the glue, and the state of the atmos- 

 phere. Meanwhile, there arc prepared three different solutions, viz : a very 

 strong solution of bichromate of potash, a solution of nitrate of silver, and 

 a weak solution of iodide of potassium. The glue is dissolved by heat, and 

 a small quantity of it is added to each of the two solutions of silver and 

 iodide. The remaining greater portion of the glue is kept warm, the solu- 

 tion of bichromate of potash added, and well mixed. After which the small 



20 



