200 Dr. Draper on some Analogies between 



and that when a plate is solarized, both free mercury and 

 amalgam are present. 



(25.) Such is the state of surface in a Daguerreotype, re- 

 cently formed. In the course of time, however, a great por- 

 tion of the mercury that is in the shadows, and also free in 

 the lights, evaporates away. When the picture has thus 

 changed, the shadows are metallic silver, and the lights silver 

 amalgam. 



(26.) 2nd. That in an iodized Daguerreotype, as taken 

 from the mercury-bath, there is no order of superposition of the 

 parts, that is to say, the iodide is neither upon nor beneath the 

 mercury, but both are as it were in the same plane. 



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 an uniform coat of yel- 

 low iodide of silver beneath. It seems, therefore, that the 

 film of iodide coheres more strongly to the metal plate than 

 the amalgam ; and further, from this result we should judge 

 that the amalgam is on the surface of the iodide. 



(27.) But this is not true; for on three different occa- 

 sions I have found that when Russian isinglass was employed 

 instead of gum, for purposes presently to be related (34.), 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 un- 

 disturbed. It is here to be understood that this action takes 

 place without the smallest disturbance of the lights and demi- 

 tints, the plate remaining in all the beauty and brilliancy 

 and perfection that it would have had, if it had been carefully 

 washed in hyposulphite of soda. 



(28.) 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 judge- 

 ment. It proves to us that the film of iodide may be me- 

 chanically torn off from the metallic surface as perfectly as it 

 can be dissolved off by chemical agents, — a singular fact. 



(29.) This result, therefore, proving that we can tear off 

 the film of iodide and leave the amalgam, can only be co- 

 ordinated with that (26.) by gum water, in which the 

 amalgam is removed and the iodide left, by supposing that 



