234 CHEMICAL RAYS AND RADIANT HEAT. [MEMOIR XVII. 



rialization is deferred. To show this effect in the ex- 

 treme, 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 hours, no effect. 



This last plate, on being submitted twice more to the 

 vapor of mercury, gave an indistinct mark. On exposing 

 a corner of it to the sun it blackened instantly, these re- 

 sults showing that the peculiar condition brought on by 

 the action of the light gradually disappears, the com- 

 pound all the time retaining its sensitiveness. 



Similar results are mentioned by Daguerre in the case 

 of the changes produced on surfaces of resinous bodies, 

 and I have noticed them in a variety of other cases. 

 Now if) whatever cause these phenomena are due, whether 

 to anything analogous to radiation, conduction, etc., it is 

 most active during the first moment after the light has 

 exerted its agency, but it must also take effect even at 

 the very time of exposure; and it is for these reasons 

 that it comes to pass that when light of a double inten- 

 sity is thrown upon a metallic plate the time required 

 to produce a given effect is less than one half. 



I could conceive the intensity of a ray so adjusted that 

 in falling upon a given sensitive preparation the loss 

 from this cause, this casting off of the active agent, should 

 exactly balance the primitive effect, and hence no observ- 

 able change result. Hereafter we shall find that one 



O 



cause of the non-sensitiveness of a number of bodies is 

 to be traced directly to the circumstance that they yield 

 up these rays as fast as they receive them. 



It needs no other observation than a critical examina- 

 tion of the sharp lines of a daguerreotype proof with a 



