CHAP, vi.] PHOTOGRAPHY. 297 



light is purely mechanical, its effects being to lift or split the coating of 

 iodide of silver, and thus to allow the mercury to come in contact with 

 the metallic silver, while the iodide that had not been split would 

 remain impervious. On examining with a microscope the mercurial 

 coating deposited after the third operation, the celebrated chemist 

 found it to be composed of very irregular granules of mercury 

 (their diameter averaging the 800th part of a millimetre). The 

 white, or luminous parts, were covered with these granules ; the 

 shadows had scarcely any ; whereas the half tints were less covered 

 than the lights : in short the granules of mercury were deposited 

 in quantities proportioned to the erosion of the iodide of silver. 



Other savants think differently According to them the iodide of 

 silver, under tue action of the luminous waves, is partially decom- 

 posed; it is transformed into sub-iodide, which, in contact with proto- 

 iodide of mercury, gives us red iodide and metallic mercury. Accord- 

 ing to this theory, which was propounded in 1843 by Messrs. 

 Ohoieelat and Ratel, ' the lights are produced by a very thin dust of 

 amalgam of silver simply deposited on the plate ; these lights are 

 brilliant in proportion to the amount of silver in this dust ; the 

 shadows are the result of a very scattered deposit of silver, mechani- 

 cally mixed with a diluted wash of mercury." 



Whichever may be the true theory, whether the granules are 

 formed of amalgam or of metallic mercury, this deposit on the sur- 

 face of the plate forms a very unstable compound, and, in either 

 case, there is the same necessity for protecting it from external 

 disturbance. Hence the importance of the gilding operation, which 

 was obtained for daguerreotype proofs, as we have seen, by the 

 deposit of a thin transparent coating of hyposulphite of gold. 



