THE CHEMICAL RAYS ARE NOT CONDUCTED. 



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



2, " in five hours, " white. 



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



4, mercurialized one hundred and forty-four, no effect. 



605. 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 of the light 

 gradually disappears, the compound all the time retaining its sensitiveness. 



606. 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 to whatever cause these phenomena are due, whether to anything analogous to 

 radiation, conduction, &c., 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 intensity is 

 thrown upon a metallic plate, the time required to produce a given effect is less than 

 one half. 



607. 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 observable change result. Hereafter 

 we shall find that one 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. 



608. It needs no other observation than a critical examination of the sharp lines of a 

 Daguerreotype proof with a magnifying glass, to show that the influence of the chemical 

 rays is not propagated laterally on the yellow iodide of silver. Of the manifestations 

 which these rays may exhibit after they have lost their radiant form and become ab- 

 sorbed, we know but little. If they conform to the analogous laws for heat, and if the 

 absorbing action of bodies for this agent is inversely as their conducting power, we 

 perceive at once why a photographic effect produced on yellow iodide of silver retains 

 the utmost sharpness without any lateral spreading ; the absorbing power is almost per- 

 fect, the conducting should therefore be zero. 



609. 3d. That, as when rays of heat fall on a mass of ice, its temperature rises degree 

 by degree, until it reaches 32 Fah., and there stops, until a certain molecular change 

 (liquefaction) is accomplished, and after that proceeds to rise again, so, also, the chemical 

 rays impress certain clianges proportional to their quantity, up to a certain point, and 

 there a pause ensues ; a very large amount of light being now rendered latent or absorbed, 

 without any indication thereof being given by the sensitive preparation (as the heat of 

 fluidity is latent to the thermometer'), a molecular change then setting in, the increments 

 of the quantity of light are again indicated by changes in the sensitive preparation. 



610. Although in the sun the iodide of silver blackens at once, this is only the result 

 of a series of preliminary operations. 



611. When we look at a Daguerreotype, we are struck with the remarkable grada- 

 tion of tint, and we naturally infer that the amount of whitening induced by mercuriali- 

 zation is in direct proportion to the amount of incident light ; otherwise it would hardly 

 seem that the gradation of tones could be so perfect. 



