MI.M..IH XVI. 1 CONDITION OF A DAGUERREOTYPE SURFACE. 225 



In Memoir XIV. I described a remarkable effect which 

 1 had noticed in these investigations, that if an object 

 such as a wafer be laid upon a piece of cold glass or 

 metal, and you breathe once on it, and as soon as the 

 moisture has disappeared remove the object and breathe 

 again on the glass, a spectral image of the wafer will 

 make its appearance. The impression thus communi- 

 cated to the surface, under certain conditions, remains 

 there a long time. During the cold weather last winter 

 I produced such an image on the mirror of my heliostat ; 

 it could be revived by breathing on the metal many 

 weeks afterwards, nor did it finally disappear until the 

 end of several months. 



I do not at present know what is the explanation of 

 this result, but the analogy between it and the arrange- 

 ment of mercurial globules covering the surface of a da- 

 guerreotype is too striking to be overlooked. It proves 

 that surfaces may assume such a condition as to affect 

 the deposition of vapors upon them, so as to produce 

 the reproduction of appearances of external forms. I 

 gave, therefore, particular attention to this point, but 

 eventually found that silver exists in an ordinary da- 

 guerreotype, in connection with the mercury all over the 

 plate, in a less proportion in the shadows, and in a 

 greater proportion in the lights. This result was, how- 

 ever, only obtained after the following fact was discov- 

 ered 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. 



Having therefore prepared three plates, D, E, F, ex- 

 actly as before, 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 sepa- 

 rated readily, and lay in chips on the surface of each 



P 



