the Chemical Rays and those of Radiant Heat. 203 



(41.) Upon an iodized plate I received a solar spectrum 

 formed by a flint-glass prism, the ray being kept motionless 

 by reflexion from a heliostat, and the plate so arranged as to 

 receive the refracted rays perpendicularly. After five minutes 

 it was mercurialized, and the resulting proof exhibited the 

 place of the more refrangible colours in the most brilliant 

 hues. The lesser refrangible colours had also left their im- 

 press of a whitish aspect, but the region of the yellow was 

 unaltered. All the different rays, therefore, except the yel- 

 low, have the power of changing this particular preparation. 

 Now, when a number of pieces of cloth of different colours 

 are placed in the sun-beam, they absorb heat in proportion 

 as their colour is deeper. A black cloth, which does not re- 

 flect any of those calorific rays, becomes presently hot ; and 

 in the same way Daguerre's sensitive preparation absorbs all 

 the rays which have any chemical action on it, and reflects 

 the yellow only, which does not affect it. In this particu- 

 lar lies the secret of its vast sensitiveness, compared with 

 the common preparations of the chloride and bromide of 

 silver. 



(4*2.) 2nd. That as a body warmed by the rays of the sun, &c. 



After a beam of light has made its impression on the 

 iodide, if the plate be laid aside in the dark before mercu- 

 rializing, that impression decays away with more or less 

 rapidity; first the faint lights disappear, then those that are 

 stronger. 



Having brought three plates to the same condition of iodi- 

 zation, and received the image of a gas-flame in the camera 

 on each for three minutes, I mercurialized one, A, forthwith ; 

 the second, B, I kept an hour, the third, C, forty-eight hours; 

 the relative appearance of these three images is represented 

 in fig. 2. 



(43.) Those who are in the habit of taking Daguerreo- 

 types, know how much they suffer when the process of mer- 

 curialization is deferred. To show this effect in the extreme, 

 I took four plates, and having prepared all alike, I exposed 

 half of the surface of each to a bright sky for eight seconds. 



No. 1 . mercurialized immediately, came out, black solarized. 



2 in five hours, white. 



3 twenty-two hours, ... same effect. 



4 one hundred and forty-four, no effect. 



(44.) This last plate, on being submitted twice more to the 

 vapour of mercury, gave an indistinct mark. On exposing 

 a corner of it to the sun it blackened instantly, these results 

 showing that the peculiar condition brought on by the action 



