138 PRESERVATION OF THE SENSITIVE PLATE INCREASES ITS SENSITIVENESS. 



zation of camphor on surfaces of dry glass, on which invisible traces have been made 

 by the pressure of a glass rod ; this also appears to belong to the same class of effects. 



528. BERZELIUS (Traite, vol. ii., p. 186) has attempted to explain (a) and (c) on 

 this principle, that the changed and unchanged surfaces radiate heat unequally. There 

 may be strong doubts with some as to the correctness of this, but is not the Daguerre- 

 otype due to the same cause, whatever it may be ? 



529. We must separate carefully the chemical changes which iodide of silver under- 

 goes in the sunbeam from the mechanical changes which happen to the sensitive film : 

 iodide of silver turns black in the solar ray ; the whole success of the Daguerreotype 

 artist depends on his checking the process before that change shall have supervened. 



530. The coating of iodine is not immediately necessary to the production of im- 

 ages by the mercurial vapour. The condition seems to be traceable to the metallic 

 surface. If you take a Daguerreotype, clean off the mercury, polish the plate thor- 

 oughly with rottenstone, wash it with nitric acid, and bring it to a brilliant surface, yet, 

 if it has not been exposed to heat, the original picture will reappear on exposure to the 

 mercurial vapour. Is not this a result of the same kind as those just referred to ? 



531. As a polishing material for the Daguerreotype plate, common rottenstone and 

 oil answer very well. The plate having been planished by the workman, is to be rub- 

 bed down to a good surface, and as high a polish given to it as possible ; it is to be 

 heated and washed with nitric acid, as indicated in the French account, and finished 

 by being rubbed with whiting (creta prceparata), in the state of a very dry powder, go- 

 ing over it for the last time with a piece of clean dry cotton ; this gives an intensely 

 black lustre, which cannot be obtained by rottenstone alone, and thoroughly removes 

 any film which nitric acid may have left. 



532. To coat with iodine, I make use of a box about two inches deep, in the bottom of 

 which that substance in coarse flakes is deposited ; no cloth intervenes, but the silvered 

 plate, with a temporary handle attached to it, is brought within half an inch of the 

 crystals, and it becomes perfectly coated in the course of from one to three minutes ; no 

 metallic strips are necessary to ensure this effect; if the edges and corners are thor- 

 oughly clean, the golden hue will appear uniformly. 



533. M. DAGUERRE recommends that the plate, after being iodized, shall be placed 

 in the camera without loss of time. The longest interval, he says, ought not to exceed 

 an hour. " Beyond this space the action of the iodine and silver no longer possesses 

 the requisite photogenic properties." 



534. There may be something peculiar in the preparation of the plate as I have de- 

 scribed it, but it is certain that this observation must be received with some limitation. 

 A plate which has been iodized does not appear so quickly to lose its sensitiveness. 

 On the other hand, by keeping it in the dark for twelve or twenty-four hours, its sen- 

 sitiveness is often remarkably increased. Other advantages also accrue. Those who 

 have made many of these photogenic experiments, will have had frequent occasion to 

 remark that the film of iodine is not equally sensitive all over; that there are spots or 

 cloudy places which do not evolve any impression ; and often the whole is in that 

 condition, that the bright parts alone come out, while the parts that are in shadow 



