ISTS.] to the BeproJiiction of Maps and P/ans. 75 



operations of printing, developing and washing can be iinislied in half an 

 hour. This process, simple as it appears, requires certain precautions in 

 using a strongly sized paper for the prints, and a very transparent original 

 to obtain the most successful results. 



It is capable of the same applications as the cyanotype last described, 

 and, like it, is quite unsuitable for producing maps for publication. 



Another process of Poitevin's is dependent on the property possessed 

 by the ferric salts of rendering gelatine insoluble, the solubility being, how- 

 ever, restored when the ferric salt is decomposed by the action of light into 

 the ferrous salt. 



Paper is thinly coated with a 6 per cent solution of coloured gelatine 

 and when dry immersed in a solution of — 



Percbloride of Iron, 10 parts or* 1 to 3 parts. 

 Tartaric acid, 3 „ „ i to 1 „ 



Water, 100 „ „ 100 



and dried in the dark. After exposure to light under a positive, such as a 

 map on tracing paper, the print is immersed in hot water, and the gelatine 

 in the parts exposed to light dissolves out, leaving an exact transcript of the 

 original drawing with dark lines on a white ground. The print may be 

 rinsed in water acidulated with hydrochloric acid to remove the iron salt. 



Salmon and Garnier have taken advantage of the fact that if paper 

 is coated with a solution of the percitrate of iron and exposed to the light, 

 the parts exposed to the light become hygroscopic in inverse proportion to 

 the intensity of the action of light, and therefore if such paper is exposed 

 to light under a map or drawing on thin paper or vellum cloth, and 

 afterwards brushed over with a fine powder, such as lamp-black or plumbago, 

 more or less of the powder will adhere to the parts protected from the light, 

 wffile the exposed parts will scarcely take it at all. After development the 

 print has only to be washed to remove the unaltered iron salt from the film. 



Other similar processes of printing with the salts of iron, uranium &c., 

 will be found in the text-books. They are, however, very little used and 

 may be regarded more as curiosities than as practical printing methods. 



Platinum printing process. — There is, however, one process which 

 deserves mention as producing very beautiful and permanent prints, in 

 which the image is formed of reduced platinum. This process has been 

 patented by the inventor, Mr. W. Willis, junior. Paper is fioated on a 

 weak solution of nitrate of silver and dried. It is then brushed over with 

 a solution of double oxalate of potassium and iron, together with a solution 

 of chloroplatinite of potassium. After exposure under a negative the print 

 is floated on a warm solution of oxalate of potash, which causes the platinum 



* Boivin, in Moniteiir ih la rhotoyraphie^ 1st April, 1878. 



