MIM..III XVII.] U1KMICAL HAYS AND RADIANT HEAT. 233 



proof exhibited the place of the more refrangible colors 

 iu the most brilliant hues. The less refrangible colors 

 had also left their impress of a whitish aspect, but the 

 region of the yellow was unaltered. All the different 

 rays, therefore, except the yellow, have the power of 

 changing this particular preparation. Now when sev- 

 eral pieces of cloth of different colors are placed in the 

 sunbeam, they absorb heat in proportion as their color 

 is deeper. A black cloth, which does not reflect any of 

 those calorific rays, becomes presently hot ; and in the 

 same way Daguerre's sensitive preparation absorbs all 

 the rays having any chemical action on it, and reflects 

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

 ticular lies the secret of its sensitiveness, compared with 

 the common preparations of the chloride arid bromide 

 of silver. 



2d. That as a body warmed by the rays of the sun, etc. 



After a beam of light has made its impression on the 

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

 curializing, that impression decays away with more or 

 less rapidity; first the faint lights disappear, then those 

 that are stronger. 



Having brought 



~ o 



three plates to the 

 same condition of 

 iodization, and re- 

 ceived 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 ap- 

 pearance of these three images is represented in Fig. 35. 



Those who are in the habit of taking daguerreotypes 

 know how much they suffer when the process of mercu- 



