200 CHEMICAL SPECTRUM. SECT. XXIV. 



tion by the same rays of discolored gum guaiacum to its 

 original tint by Dr. YVollaston, 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 be- 

 yond and at the red extremity of the solar spectrum, in 

 most instances, 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 

 knowledge 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 colors very nearly cor- 

 responding with its natural hues. But a very remarka- 

 ble phenomenon occurred at the end of least refrangi- 

 bility ; the red rays exerted a protecting influence 

 which preserved the paper from the change which it 

 would otherwise have undergone from the deoxydizing 

 influence of the dispersed light which always surrounds 

 the solar spectrum, and this maintained its whiteness. 

 Sir John met with another instance on paper prepared 

 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 main- 

 tained the whiteness of the paper to a considerable dis- 

 tance beyond the last traceable limit of the visible rays, 

 thus evincing decidedly the existence of some chemical 

 power over a considerable space beyond the least re- 

 frangible end of the spectrum. Mr. Hunt also found 

 that on the Daguerreotype plate a powerful protecting 

 influence is exercised by the extreme red rays. In 

 these cases the red and those dark rays beyond them 

 exert an action -of an opposite nature to that of the violet 

 and lavender ravs. 



