1878.] to the JRcproduction of Maps and Plans. 73 



cases where silver printing is required for maps &c. It is more economical 

 than the ordinary process, and much more rapid in working, the exposure 

 heing counted by seconds instead of by minutes. The operations are briefly 

 as follows : 



Thin photographic paper is salted by floating on a solution containing 

 2 per cent each of chloride of ammonium and citrate of soda in water 

 slightly acidified with citric acid. The paper is dried and may be kept for 

 use. It is sensitised by floating in the dark on a bath containing 5 per cent 

 of nitrate of silver acidified with a little citric acid. The exposure to light 

 is conducted in the same way as in the ordinary process, but is exceedingly 

 sl^ort — a few seconds to one minute being generally sufficient. When taken 

 from the printing frame only a very feeble image is visible, it is therefore 

 developed in a mixture of a solution of gallic acid (1 to 3800 water) with 

 a solution of acetate of lead (I to 200 water), to which a little acetic acid is 

 added. The detail gradually strengthens, and in about a quarter of an hour 

 the prints are fully developed and of a good black colour. After being 

 w^ashed they are fixed in a bath of hyposulphite of soda at 30 \)QV cent., 

 then well washed and dried.* 



The foregoing are the principal methods of silver printing suitable for 

 cartographic purposes, but, however convenient and useful they may be for 

 special objects where photolithography is not applicable, they cannot be 

 considered adapted for purposes of publication, and their want of perman- 

 ency is an insuperable defect. 



Printing with Salts of Iron. — From time to time attention has been 

 drawn to the possibility of replacing silver-printing by processes depending 

 on the use of the salts of iron and other cliea]) materials, but, though 

 certainly useful in some res^^ects, they have never been brought into 

 extensive practical use. 



"• One of the best known of these processes is the ' cyanotype', invented 

 by Sir John Herschel, and lately re-introduced by Messrs, Marion & Co., 

 of Paris, who prepare and supply the ferro-prussiate paper ready for use. 

 Good even-textured paper is brushed over with a mixture containing 

 nearly equal proportions of 10 or 12 per cent solutions of ammonio- citrate 

 of iron and the ferridcyanide of potassium, dried and exposed to light 

 under a print or drawing placed with the printed side uppermost. The 

 resulting faint photographic image is developed and fixed by a mere wash- 

 ing in plain water, yielding a print in white or light blue lines on a dark 

 blue ground. 



This process is rapid, simple and cheap. The camera is dispensed 

 with, and the only photographic apparatus required is a printing frame and 



* Maes and Hannot's Traiic do Topographic ct de Reproduction den Cartes au motjen 

 de la Thotographie^ p. 29.5. 



