IODINE IS NOT EVOLVED, BUT CORRODES THE PLATE. 



Soon after I had ascertained the action of gum-arabic, some of it was applied to the 

 surface of a plate on which an impression had just been formed in the mercury- 

 bath. This was without removing the coat of iodine. On drying it, the gum chipped 

 up, as was expected, bringing away with it all the lights of the picture, and leaving 

 a uniform coat of yellow iodide of silver beneath. It seems, therefore, that the film 

 of iodide coheres more strongly to the metal plate than the amalgam ; and, farther, 

 from this result we should judge that the amalgam is on the surface of the iodide. 



584. But this is not true ; for on three different occasions I have found that when 

 Russian isinglass was employed instead of gum, for purposes presently to be related 

 (591), the isinglass, from its stronger cohesive power, chipped off in the act of drying, 

 tearing up the yellow film from end to end of the plate, and leaving the amalgam, 

 constituting the lights, undisturbed. It is here to be understood that this action takes 

 place without the smallest disturbance of the lights and demitints, the plate remain- 

 ing in all the beauty, and brilliancy, and perfection that it would have had if it had 

 been carefully washed in hyposulphite of soda. 



585. This is a result, however, which I cannot produce with uniformity. Most 

 commonly, the lights are torn up with the iodide. Had it occurred but once, I should 

 still have cited it with decision, for, from the very character of it, it is impossible 

 to be mistaken, or to commit an error of judgment. It proves to us that the film 

 of iodide may be mechanically torn off" from the metallic surface as perfectly as it can 

 be dissolved offhy chemical agents a singular fact. 



586. This result, therefore, proving that we can tear off the film of iodide and leave 

 the amalgam, can only be co-ordinated with that (583) by gum-water, in which the 

 amalgam is removed and the iodide left, by supposing that there is not anything like 

 a direct superposition in the case, and that the particles of amalgam and iodide lie, 

 as it were, side by side. 



587. 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 Daguerreotype the light communicates a ten- 

 dency 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 Jirst ; and this has happened 

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

 mercurial operation. 



There is no difficulty in proving this directly, and the indirect evidence is copious. 

 If we lay a piece of paper imbued with starch on an iodized plate, and expose it to 

 the sun, although the plate presently assumes a dark olive-green colour, the starch re- 

 mains uncoloured. 



588. This dark substance is probably a subiodide of silver ; the iodine, therefore, 

 which has been disengaged from it not having been set free, must have necessarily united 

 with the adjacent metallic silver : this, for very obvious reasons, there is no difficulty 

 in admitting. 



