the Chemical Rays and those of Radiant Heat. 199 



(20.) I do not at present know what is the reason of this 

 result, but the analogy between it and the arrangement of 

 mercurial globules, which cover the surface of a Daguerreo- 

 type, is too striking to be overlooked. It proves to us that 

 surfaces may assume such a condition as to affect the deposi- 

 tion of vapours upon them, so as to give rise to the repro- 

 duction of appearances of external forms. I gave therefore 

 particular attention to this point, but eventually found that 

 silver exists in an ordinary Daguerreotype, in connexion with 

 the mercury all over the plate, in a less proportion in the 

 shadows, and in a greater proportion in the lights. This re- 

 sult was, however, only obtained after the following fact was 

 discovered — that the mucilage of gum-arabic, when slowly 

 dried in a thin layer on the surface of a Daguerreotype, splits 

 up in shivers, bringing along with it the white portions of the 

 picture, and leaving the plate clean. 



(21.) Having therefore prepared three plates, D, E, F, ex- 

 actly as before (16.), I poured on them a solution of gum, 

 drained them so as to leave only a small quantity, and let 

 them dry slowly over the sand-bath. The gum separated 

 readily, and lay in chips on the surface of each 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 : — 



(22.) 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 muriatic acid was now 

 added, and the chloride of silver immediately fell. In re- 

 peating 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 chloride of silver generated. 



(23.) As, from the minuteness of that quantity, it was im- 

 possible 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 sometimes the other appearing to have a slight ad- 

 vantage. 



(24.) I conclude, therefore, that whilst 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 demitints ; 



