198 Dr. Draper on some Analogies between 



I iodized D in the dark, and mercurialized it forthwith at 

 170° Fahr., taking the utmost care that not a ray of light 

 should be suffered to impinge upon it. 



E was iodized, and exposed for two minutes to^ diffused 

 daylight, and then mercurialized at 170° Fahr. 



F was iodized, and exposed to the sun until it began to 

 turn brown, an effect occurring almost at once. It was then 

 mercurialized at 170° Fahr. 



All these plates, then, had their sensitive coating removed 

 by hyposulphite, and were thoroughly washed in distilled 

 water and dried. 



(17.) I had therefore three plates, representing accurately 

 the conditions proposed to be investigated. D was in the 

 condition of the most perfect shadows, E in that of the 

 highest lights, and F solarized. In appearance D was black, 

 E was white, and F bluish-gray. 



Upon D, E, F, I placed A, B, C respectively, separating 

 each pair of plates one-sixteenth of an inch, or thereabouts, 

 by slips of glass. Then I laid them on the level surface of 

 the sand-bath, the test plates being kept cool by sponging oc- 

 casionally with water. Temperature of the sand 200° Fahr., 

 duration of the experiment fifteen minutes. 



On examination, A, B, C were all found powerfully mer- 

 curialized, nor did there seem to be any difference between 

 them. 



(18.) I consider, therefore, that the shadows, the demitints, 

 the lights, and tbe solarized portions of a Daguerreotype, are 

 covered with mercury ; for at a] temperature of 200° Fahr. 

 they all evolve it alike, a sufficiency of vapour rising from the 

 parts that have not been exposed to the light, to bring a plate 

 that has been so exposed to its maximum of whiteness*. 



(19.) In a former Number of this Journal (s. 3. vol. xvii. 

 p. 218), I described a remarkable effect which I 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 commu- 

 nicated to the surface, under certain conditions, remains there 

 for a long time. During the cold weather last winter, I pro- 

 duced such an image on the mirror of my heliostate ; it could 

 be revived by breathing on the metal many weeks afterwards, 

 nor did it finally disappear until the end of several months. 



* I believe that the most delicate test for the presence of mercury, is 

 a slip of silver iodized to a yellow colour, and exposed for two or three 

 minutes to a weak daylight. 



