ON THE DAGUERREOTYPE PROCESS. 145 



4th. That it depends on the CHEMICAL nature of the ponderable material what rays 

 shall be absorbed. 



5th, That while the specific rays thus absorbed depend upon the chemical nature of 

 the body, the absolute ainount is regulated by its OPTICAL qualities, such as depend on 

 the condition of its surfaces and interior arrangement. 



6th. It will be proved from this, that the SENSITIVENESS of any given substance de- 

 pends on its chemical nature and optical qualities conjointly, and that it is possible to 

 exalt or diminish the sensitiveness of any given chemical compound, by changing the 

 character of its optical relations. We shall here meet with an explanation of some of 

 the tacts noticed by Sir J. HERSCHEL, Mr. HUNT, Mr. TALBOT, and others, respecting 

 the increase of sensitiveness of the chloride of silver and other bodies. 



7th. That, as when radiant heat falls on the surface of an opaque body, the rays 

 reflected are complementary in number to those that are absorbed, so, in the case of a 

 sensitive preparation, the rays reflected are complementary in number to those that 

 are absorbed. 



570. OF THE DAGUERREOTYPES. 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. 



-?<! That in an iodized Daguerreotype, as taken from the mercury-bath, there is 

 no order of superposition of the parts, that is to say, the iodide is neither upon nor 

 beneath the mercury, but both are, as it were, in the same plane. 



3d. That when a ray of light falls upon the surface of this preparation, through all 

 the intervening steps, and up to the point of maximum action, no iodine is evolved 

 from the plate ; but that in the common Daguerreotvpe the light communicates a ten- 

 dencv to the atoms of the iodide to yield up to the mercurial vapour their silver, while 

 the iodine retires and combines with the unaffected silver around. It follows that when 

 such a plate is withdrawn from the mercurial vapour, there is all over it a uniform film 

 of iodide of silver of the very same thickness as at first ; and this has happened through 

 a direct corrosion of the silver by the iodine, while it was undergoing the mercurial 

 operation. 



571. I pass at once to the proofs of these several propositions, commencing, for the 

 sake of perspicuity, with those relating to the Daguerreotype first: and 1st, That me- 

 tallic 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. 



572. I took a plated copper three inches by four in surface, and having prepared it 

 with care, I exposed half of it to the diffused light of the day, screening the other half; 

 it was then mercurialized at 175 Fah., the iodide removed by hyposulphite of soda 

 and washed. And now a plate on which a gold leaf was spread was placed over it, 

 but separated, as shown in fig. 94, in the points , b, c, by three slips of glass. By 

 means of a spirit-lamp the photographic plate a, b, c, was heated, and the gilded plate 



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