226 CONDITION OF A DAGUERREOTYPE SURFACE. [MEMOIK XVI. 



plate; it was easily removed to three sheets of paper, by 

 tapping with the finger on the back of the plate. Each 

 was then treated alike as follows : 



The gummy matter was incinerated on a platinum 

 leaf, and the remaining ashes transferred to a test-tube 

 half an inch in diameter. One drop of nitric acid and 

 one drop of water were added ; it was boiled over a 

 small flame, and diluted with a little water. Dilute hy- 

 drochloric acid was now added, and the chloride of silver 

 immediately fell. In repeating this, it is necessary to 

 attend to the state of dilution of the acid, for if too 

 strong it wholly dissolves the minute quantity of chlo- 

 ride of silver generated. 



As, from the minuteness of that quantity, it was 

 impossible to obtain a direct quantitative analysis, I 

 adopted the foregoing method, and added the dilute 

 acid to all three tubes at the same time. In D there 

 was a faint opalescence, in E and F a cloud ; but I 

 could not always determine whether the deposit of E 

 or F was most copious, sometimes the one and some- 

 times the other appearing to have a slight advantage. 



I conclude, therefore, that while the whole surface of 

 the plate is coated with mercury, it exists as silver amal- 

 gam chiefly in the lights, and as uncombined mercury 

 chiefly in the shadows, and in a mixed proportion in the 

 demi-tiuts; and that when a plate is solarized, both free 

 mercury and amalgam are present. 



Such is the state of surface in a daguerreotype, recently 

 formed. In the course of time, however, a great portion 

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

 the lights, evaporates aw r ay. When the picture has thus 

 changed, the shadows are metallic silver, and the lights 

 silver amalgam. 



2d. That in an iodized daguerreotype, as taken from 

 the mercury -bat] i, there is no order of superposition of the 



