CHEMICAL AGENCIES OF LIGHT. 101 



Iii accordance with the foregoing facts, it has 

 been found that each of the primitive rays pro- 

 duces specific changes of colour in different bo- 

 dies; that the blue and violet rays of the spectrum 

 change the white chloride of silver to a dark 

 violet hue; whereas the red rays change it to a 

 rose colour; and that the same change of colour 

 is produced on the protochloride of lead and the 

 oxide of mercury, when moistened and exposed 

 to the red rays of the spectrum, as in the experi- 

 ments of Scheele, Sennebier, Davy, and Seebeck: 

 that a piece of white paper stained yellow with 

 a solution of guiacum in alcohol, was turned 

 green in five minutes by exposure to sunshine, 

 but remained unaltered for several months in the 

 dark ; and that when the yellow paper was made 

 green by exposure to the violet rays, its original 

 colour was restored by exposure to the red rays 

 of the spectrum, as shown by the experiments of 

 Wollaston. 



The chemical agency of light is still more 

 strikingly illustrated in the production of those 

 accurate pictorial delineations of objects termed 

 Daguerreotypes, which are formed by the imme- 

 diate influence of the regal sun. Moreover, that 

 there is " a specific affinity between definite 

 atoms and definite rays," is evident from the 

 fact, that bodies repel and reflect certain rays, 

 while they attract and absorb others. For ex- 

 ample, it was found by Sir David Brewster, 

 that red transparent solutions and glasses per- 



