COLOR PHOTOGRAPHY. 175 



I designate as the Seebeck process in general that in which the 

 sensitive material is i)reliminarily exposed chloride of silver powder. 1 

 I used in this pure chloride of silver precipitated in the dark and then 

 dried. The powder was then placed between two glass plates and the 

 edges of these cemented together. The preliminary exposure was 

 made at first with violet and ultraviolet, but later more quickly with 

 white light. It was continued until the powder had taken on a not 

 too dark violet color. 



Becquerel 2 has experimented with various modifications. I used 

 exclusively and distinguish here as the Becquerel process that peculiar 

 to him, and employed electrolytically cklorinized silver plates, but with- 

 out subsequent heating. For their preparation I used brightly polished 

 electrolytically silvered copper or brass plates or else thin sheets of silver 

 themselves. There were dipped in a weak solution of hydrochloric acid 

 (1 :8) as positive electrode, while a current of from two to four amperes 

 passed between surfaces of about 80 square centimeters area for some 

 seconds. The thickness of silver chloride deposit recommended by 

 Becquerel as the most satisfactory was attained by the passage of a quan- 

 tity of electricity which would suffice to separate 0.067 cubic centimeters 

 of hydrogen per square centimeter of silver surface. This thickness 

 is according to an approximate calculation about 0.0016 millimeters. 

 After coating, the plate is quickly dried between filter papers, and theu 

 rubbed with soft leather. 



Poitevin's 3 process was used by Zenker and Krone and developed by 

 them. 4 Following their directions I bathed Bives-Rohpapier in a 10 per 

 cent solution of common salt for two minutes, then for one minute in an 

 8 per cent solution of nitrate of silver. The leaf was then quickly 

 washed and was exposed in diffused daylight to the action of a stannous 

 chloride solution containing 5 grams of stannous chloride to 100 cubic 

 centimeters of water, till it had attained a not too dark coloration. 

 After this it was bathed in a mixture of one part concentrated potassium 

 chromate solution and two parts concentrated copper sulphate solution 

 and preserved between filter papers. It is well to moisten the paper 

 somewhat before exposure if quite dry. 



Development is of course unnecessary in any of these processes, as 

 the colors are formed during the exposure. 



Fixing, which in the last of these methods is possible to a slight 

 extent, I have not undertaken. 



1 See citation, p. 168. All these processes are described in the books of Zenker 

 (see p. 225) and Krone (see p. 227). 



2 Edmond Becquerel, Annales de chiinie et de physique (3), 22: page 451, 1848; 

 25: page 447, 1849; 42: page 81, 1854 ; see also E. Becquerel, "LaLurniere," 2: page 

 209. Paris, Firmin Didot Freres, Fils et Cie, 1868. 



3 Poitevin. Comptes rendus, 61: page 1111. 1865. 



4 See the work already mentioned. 



