SECT, xxiv.] CHEMICAL SPECTRUM. 231 



restoration by the same rays of discoloured gum guaiacum 

 to its original tint by Dr. Wollaston, have already been men- 

 tioned as giving the first indications of that difference in 

 the mode of action of the chemical rays at the two ends of 

 the visible spectrum, now placed beyond a doubt. 



The action exerted by the less refrangible rays beyond 

 and at the red extremity of the solar spectrum, in most in- 

 stances, so far from blackening metallic salts, protects them 

 from the action of the diffused daylight : but, if the prepared 

 surface has already been blackened by exposure to the sun, 

 they possess the remarkable property of bleaching it in some 

 cases, and under other circumstances of changing the black 

 surface into a fiery red. 



Sir John Herschel, to whom we owe most of our know- 

 ledge of the properties of the chemical spectrum, prepared 

 a sheet of paper by washing it with muriate of ammonia, 

 and then with two coats of nitrate of silver ; on this surface 

 he obtained an impression of the solar spectrum exhibiting 

 a range of colours very nearly corresponding with its natural 

 hues. But a very remarkable phenomenon occurred at the 

 end of least refrangibility ; the red rays exerted a protect- 

 ing influence which preserved the paper from the change 

 which it would otherwise have undergone from the deoxy- 

 dizing influence of the dispersed light which always sur- 

 rounds the solar spectrum, and this maintained its white- 

 ness. Sir John met with another instance on paper pre- 

 pared with bromide of silver, on which the whole of the 

 space occupied by the visible spectrum was darkened down 

 to the very extremity of the red rays, but an oxydizing 

 action commenced beyond the extreme red, which maintained 

 the whiteness of the paper to a considerable distance beyond 

 the last traceable limit of the visible rays, thus evincing 

 decidedly the existence of some chemical power over a con- 

 siderable space beyond the least refrangible end of the 

 spectrum. Mr, Hunt also found that on the Daguerreotype 



