208 Dr Fyfc oii Daguerreotype. 



it retain their original golden hue. When the plate is long 

 exposed, on the parts on which the light impinges, the yellow 

 is changed to dark violet, and, by shght friction, a dark co- 

 lom-ed powder is removed. When this is treated with water 

 or alcohol, the fluids do not, on the addition of the usual tests, 

 indicate the presence of uncombined iodine. The iodinized 

 plate, after long exposure to light, so as to darken it, does not, 

 when heated, give off vapour of iodine ; nor is its svirface at 

 all changed, from which it is evident that the powder libe- 

 rated from the plate, by the agency of the light on the iodid, is 

 not free iodine. Most probably it is the iodid set free from the 

 silvei', but the quantity procured is so very minute that I have 

 not been able to satisfy myself as to its chemical composition. 



The most important question regarding this interesting 

 subject refers to the last stages of the process — the mercurial- 

 izing and washing with hyposulphite. 



When the plate is removed from the camera, and without 

 any image observed on it, and is then exposed to the vapom* 

 of mercm-y, the mercm-y evidently adheres to those parts only 

 which have been acted on by the light. Of course, the greater 

 the action of the light, the more is the iodid acted on, and the 

 thicker is the mercurial coating acquired, or the more readily 

 does the mercury adhere to the silver ; thus giving different 

 degrees of density, or a difference of surface, while those parts 

 on which the light has not impinged still retain their golden- 

 yellow hue. 



On washing with hyposulphite, the whole of the iodid is 

 removed, and silver and mercurial surfaces are then exposed ; 

 and it is the contrast betv/een them, and not any particular 

 chemical action exerted on the silver, by the hyposulphite, that 

 produces the effect. In fact, it is a mere difference in reflec- 

 tion of light from a highly resplendent silver plate, and from 

 a dull mercurialized surface ; the former presenting the dark, 

 the latter the light parts of the picture ; the variety of shades, 

 from dark to light, being brought out by a difference in the 

 density, or by inequality in the mercurialized parts, accord- 

 ing to the action of the light ; for we can easily conceive 

 that, the less and less the action of light, the thinner and 

 thinner, or more imperfect, may be tlie mercurial coating, 



