the Chemical Rays and those of Radiant Heat. 1 97 



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. 



(13.) 3rd. 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 Daguerreotype the light 

 communicates a tendency to the atoms of the iodide to yield 

 up to the mercurial vapour their silver, whilst the iodine re- 

 tires and combines with the unaffected silver around. It fol- 

 lows, that when such a plate is withdrawn from the mercurial 

 vapour, there is all over it an uniform film of iodide of silver, 

 of the very same thickness as at first, and that this has hap- 

 pened through a direct corrosion of the silver, by the iodine, 

 whilst it was undergoing the mercurial operation. 



(14.) I pass at once to the proofs of these several proposi- 

 tions, commencing, for the sake of perspicuity, with those re- 

 lating to the Daguerreotype first : and 1st, That metallic 

 mercury exists all over the surface of an ordinary Daguerreo- 

 type, in the shadows as isoell as in the lights, — in the shadows it 

 is as metallic mercury, in the lights as silver amalgam. 



(15.) I took a plated copper three inches by four in sur- 

 face, 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° Fahr., 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 sepa- 

 rated, as shown in Plate I. fig. 1., in the points a, b, c by 

 three slips of glass. By means of a spirit-lamp the photo- 

 graphic plate a, b, c was heated, and the gilded plate g k kept 

 cool, by occasionally wetting it. On parting the plates, it was 

 perceived that faint but distinct traces of whitening were vi- 

 sible all over the gold, as well on that part which was over 

 the whitened half of the photograph, as over that which was 

 unchanged. 



(16.) But as it might happen that the mercury diffused it- 

 self laterally past the imperfect obstacle b, I made the follow- 

 ing decisive trials : — 



I iodized three silver plates, A, B, C, each three inches by 

 four in surface, conducting the processes for each in the same 

 way ; and having exposed each for two minutes to a faint day- 

 light, I laid them aside in the dark, to be presently used as 

 test plates, in lieu of the gilded plate (g, Jc). 



Then I took three other plates, D, E, F, of the same size, 

 and conducting the preparatory processes for each as before, 



