212 THE DAGUERREOTYPE. [MEMOIR XIV. 



iodized surface, but the chief use of an angle of 45 is to 

 allow the operator to inspect the process through the 

 glass of the mercury-box. 



Sometimes it is advantageous to heat the mercury a 

 second time, when the proof is not distinctly evolved at 

 first. Indeed, it occasionally happens that a proof which 

 did not evolve at all at first will come out quite fairly 

 on raising the temperature of the mercury again. 



M. Daguerre recommends two methods of removing 

 the sensitive coating from the plate by washes of hypo- 

 sulphite of soda, and by a solution of common salt. The 

 former answers perfectly, the second only indifferently 

 well. There is, however, another process, which is very 

 simple, and has an advantage over the former of these in 

 cheapness. It adds not a little to the magic of the whole 

 operation, in the eyes of those who are unaccustomed to 

 chemical results. The plate, having been dipped into 

 cold water, is placed into a solution of common salt, of 

 moderate strength ; it remains without being acted upon 

 at all; but if it be now touched on one corner with a 

 piece of zinc, which has been scraped bright, the yellow 

 coat of iodide moves off like a wave and disappears. It 

 is a very pretty process. The zinc and silver forming 

 together a voltaic couple, with the salt water intervening, 

 oxidation of the zinc takes place, and the silver surface 

 commences to evolve hydrogen gas; while this is in a nas- 

 cent condition it decomposes the film of iodide of silver, 

 giving rise to the production of hydriodic acid, which is 

 very soluble in water, and hence instantly removed. 



This process, therefore, differs from that with hyposul- 

 phite. The latter acts by dissolving the iodide of silver, 

 the former by decomposing it. It is necessary not to 

 leave the zinc in contact too long, or it deposits stains, 

 and in large plates the contact should be made at the 

 four corners successively, to avoid this accident. 



