On the principal Phcenomena of Photography. 375 



images having the same'appearance as those developed under 

 the action of mercurial vapour. This direct and immediate 

 effect of light is certainly remarkable ; but the Daguerreotype 

 process is not founded on that principle on account of the 

 slowness of its action ; and it is fortunate that, long before 

 light can produce the white precipitate I have alluded to, it 

 operates another effect, which is the wonderful property of 

 attracting the vapour of mercury. This vapour is condensed 

 in the form of a white powder, having also, when examined by 

 the microscope, the appearance of reflecting crystals. The 

 Daguerreotype image is due to this property, which is the 

 most beautiful feature of Daguerre's discovery. 



M. Moser has given an ingenious theory of the action of 

 mercury. Knowing that the yellow ray had the property of 

 continuing the effect commenced by light on the iodide of 

 silver, he has supposed that mercury, when in a state of va- 

 pour, evolves a latent yellow light, and to the action of that 

 yellow light of mercurial vapour he ascribes the continuation 

 of die decomposition of the iodide of silver. But as the ana- 

 lysis of the surface discloses the presence of mercury, that 

 metal must have been amalgamated with the silver set free 

 after the action of light. We must therefore look for another 

 explanation of the phaenomenon. 



It is more probable that light exercises a twofold action on 

 the iodide of silver, whether it is combined or not with chlorine 

 or bromine. By one, the iodide is decomposed, and the silver 

 set free is precipitated on the surface in the form of a white 

 powder or small crystals ; by the other, which begins long be- 

 fore the former, the parts affected by light have been endowed 

 with an affinity for mercurial vapour. • 



By means of my photographometer, to the principle of 

 which I shall presently refer, 1 have been able to ascertain 

 that the pure light of the sun performs in about two or three 

 seconds the decomposition of the bromo-iodide of silver, which 

 is manifested by the white precipitate ; while the same inten- 

 sity of light determines the affinity for mercurial vapour in the 

 wonderfully short space of about joV o^^^ P^^*^ "^^ second. So 

 that the affinity for mercury is imparted by an intensity of 

 light 3000 times less than that which produces the decom- 

 position manifested by the white precipitate. 



For this reason it is difficult to suppose that the two actions 

 are the same. We must admit that they are different. Long 

 before it can effect the decomposition of the surface, light 

 imparts to the sensitive coating the affinity for mercurial 

 vapour ; and this appears to be the principle of the formation 

 of the image in the Daguerreotype process. 



