CHEMICAL INFLUENCE OF LIGHT. 279 



Chloride of Nitrate of Chloride of Nitrate of 



sodium. silver. silver. sodium. 



NaCl + AgN0 3 = AgCl + NaNO 3 



Strips of paper thus prepared, placed in parts of the spec- 

 trum where the chemical power resides, will be darkened, 

 because the chloride of silver is decomposed, the chlorine 

 passing off and leaving the silver attached to the fibres of 

 the paper. It is to be remembered in preparing these pa- 

 pers that exposure to the light has the same effect upon 

 them that placing them in the chemical limits of the spec- 

 trum does, for every ray of white light has the chemical 

 power bound up in it. For this reason the papers must 

 be prepared in a dark room, and, after being dried by blot- 

 ting-paper, must be put between the leaves of a book to 

 prevent the light from coming to them. 



Some other experiments akin to these may be tried with 

 colored glasses. Glass stained dark blue with oxide of co- 

 balt lets scarcely any light pass through, but offers no hin- 

 derance to the passage of actinism, as may be seen by using 

 the papers charged with chloride of silver. Yellow glass, 

 on the other hand, will let the light and heat pass, but not 

 the actinism. You remember that a mixture of hydrogen 

 and chlorine exposed to the direct light of the sun explodes, 

 so rapid is their union, while they do not unite at all if the 

 mixture be kept in the dark. Now when the mixture of 

 the two gases is exposed to the sun in a vessel or tube of 

 red glass scarcely any effect is produced ; but if it be ex- 

 posed in a tube of violet-colored glass the gases combine 

 rapidly with an explosion, just as they do when the glass 

 is without color. 



395. Light-Pictures. If lace be spread over paper charged 

 with chloride of silver, on exposure to light for a few min- 

 utes its whole shape, to the minutest thread, will be traced 

 in white lines. The explanation is this: The chemical 



