274 CHEMISTRY. 



CHAPTER XXL 



CHEMICAL INFLUENCE OP LIGHT. 



388. Chemical Influence of Light. You have already had 

 some illustrations of the fact that the rays of the sun not 

 only give light and heat to the earth, but also stimulate 

 many chemical operations. For example, you saw in 223 

 that chlorine and hydrogen are very ready to unite under 

 the stimulus of light, when, if light be shut out, no such 

 union takes place. So strong is this disposition to unite 

 under this stimulus, that if a mixture of the two gases be 

 exposed to the direct rays of the sun, the union is so sud- 

 den as to occasion an explosion. A solution of ferrous 

 sulphate may be kept a long time in the dark without any 

 change ; but expose it to sunshine, and a precipitation of 

 ferric oxide at once begins. Indeed, it is ascertained that 

 precipitation in many cases may be quickened by the rays 

 of the sun. You have a familiar example of the chemical 

 influence of solar light in the blackening of common mark- 

 ing-ink when the marked articles are exposed to the light. 

 In sun-bleaching, also, the sun's rays stimulate the chemical 

 changes which take place. 



389. Universality of this Influence. Wherever light goes 

 it acts chemically. It was said by Niepce, who was asso- 

 ciated with Daguerre in the investigations which led to 

 his great discovery, that " no substance can be exposed to 

 the sun's rays without undergoing a chemical change." 

 Though, with the common notion which was prevalent that 

 the sun, the great source of light and heat, had little to do 



