298 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



CHAPTER VII. 



PHOTOGRAPHY ON PAPER AND ON GLASS. 



I. PHOTOGRAPHY ON PAPER. TALBOT'S INVENTION. BLANCQUAP.P- 

 EVRARD PROCESSES. 



As the names of Niepce and Daguerre are associated with the first 

 invention of photography on metal plates, so those of Talbot and of 

 Blancquard-Evrard characterize the discovery of photography on 

 paper : Niepce and Talbot 1 having the glory of conceiving the idea ; 

 Daguerre and Blancquard-Evrard of having practically realized and 

 perfected the process of the original inventor. 



Less than two years had elapsed since Francois Arago and Gay- 

 Lussac had made their reports in the Chamber of Deputies and in the 

 Chamber of Peers on the invention of the daguerreotype, when a 

 letter from an English scientific man, Fox-Talbot, read at the Academy 

 of Sciences by Biot, explained the processes he had discovered for 

 reproducing images directly on sensitized paper. According to this 

 communication Talbot's process is as follows : 



" With a solution of nitrate of silver in pure water wash one of the 

 sides of a sheet of paper, previously marked to recognize it, and then dry 

 gradually. After this plunge it for two minutes into a solution of iodide 

 of potassium. By mixing a solution of nitrate of silver with a solution of 

 gallic acid and a small quantity of acetic acid, gallonitrate of silver is 

 formed, in which the iodized paper must be washed. The paper thus 

 saturated is then plunged in w r ater and dried with blotting paper, and 



1 Niepce and Talbot, in June 1839, six months previous to the publication of the 

 daguerreotype process, read a paper before the Royal Society, giving an account of 

 a process of photographic printing which is similar in its main outline to that prac- 

 tised at the present time. 



