PHOTOGRAPHS. 329 



837. CHEMICAL ACTION OF LIGHT. The 



What is said . . . . 



of theckemi- chemical action or light, on which the 

 t production of daguerreotypes depends, is 



rays possess one o f tne mO st interesting and remarkable 



this power ? 



of chemical phenomena. The rays of 

 the sun are so subtle, that they pass through solid crys- 

 tal and leave no trace of their passage. Yet with them 

 comes a power that can overcome the strongest chemical 

 affinities, and resolve the compounds which it has pro- 

 duced into their original elements. This power resides 

 in what are called the chemical, act wic, or tit/ionic rays. 

 These are mingled, under ordinary circumstances, with 

 those of light, but are capable of separation by certain 

 media. 



What are pho- $38. PHOTOGRAPHS. - Pictures produced 



tographs ? through the agency of light, whether upon 

 silver, or paper, are, properly, photographs, or light pic- 

 tares ; the name, however, is especially appropriated 

 to the latter. For the purpose of illustration, a method 

 of producing negative pictures, as they are called, will 

 be here given. 



H is scnsi- 839 - The sensitive paper required in 



tive paper the process, is prepared by floating a slip of 

 letter-paper, for two or three minutes, upon 

 salt water ; and then for double the time, with the same 

 side down, on a solution of nitrate of silver. Chlo- 

 ride of silver forms within the fibres, and renders the 

 paper sensitive to light. After each immersion, the slip 

 should be dried off by blotting paper. When finished, it 

 should be immediately laid away between the leaves 

 of a book, for protection against the light. 



