1 96 Dr. Draper on some Analogies between 



(5.) 2nd. That as a body warmed by the rays of the sun 

 gradually loses its heat by radiation, or conduction, or con- 

 tact with other bodies, so likewise, by some unknown process, 

 photographic effects produced on sensitive surfaces are only 

 transient, and gradually disappear. 



(6.) 3rd. That, as when rays of heat fall on a mass of cold 

 ice, its temperature rises degree by degree, until it reaches 

 32° Fahr., 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 changes 

 proportional to their quantity, up to a certain point, and there 

 a pause ensues ; — a very large amount of light being now ren- 

 dered latent or absorbed, without any indication thereof be- 

 ing given by the sensitive preparation (as the heat of fluidity 

 is latent to the thermometer) — a molecular change then set- 

 ting in, — the increments of the quantity of light are again in- 

 dicated, by changes in the sensitive preparation. 



(7.) 4th. That it depends on the chemical nature of the 

 ponderable material what rays shall be absorbed. 



(8.) 5th. That whilst the specific rays thus absorbed de- 

 pend upon the chemical nature of the body, the absolute 

 amount is regulated by its optical qualities, such as depend 

 on the condition of its surfaces, and interior arrangement. 



(9.) 6th. It will be proved from this, that the sensitive- 

 ness of any given substance depends on its chemical nature 

 and optical qualities conjointly, and that it is possible to exalt 

 or diminish the sensitiveness of any given chemical com- 

 pound, by changing the character of its optical relations. 

 We shall here meet with an explanation of some of the facts 

 noticed by Sir J. Herschel, Mr. Hunt, Mr. Talbot, and 

 others, respecting the increase of sensitiveness of the chloride 

 of silver and other bodies. 



(10.) 7th. That, as when radiant heat falls on the surface 

 of an opake body, the number of rays reflected is the com- 

 plement of those that are absorbed, so in the case of a sen- 

 sitive preparation, the number of rays reflected from the sur- 

 face is the complement of those that are absorbed. 



Of the Daguerreotypes. 



(11.) In relation to the condition of these tablets, I shall 

 prove the following facts : — 



1st. That metallic mercury exists all over the surface of an 

 ordinary Daguerreotype, in the shadows as well as in the 

 lights, — in the shadows it is as metallic mercury, in the lights 

 as silver amalgam. 



(12.) 2nd. That in an iodized Daguerreotype, as taken from 



