146 SPECTRAL IMAGES. 



g k kept cool, by occasionally wetting it. On parting the plates, it was perceived that 

 faint but distinct traces of whitening were visible all over the gold, as well on that part 

 which was over the whitened half of the photograph as over that which was unchanged. 



573. But as it might happen that the mercury diffused itself laterally past the imper- 

 fect obstacle b, I made the following decisive trials : 



I iodized three silver plates, A, B, C, each three inches by four in surface, conducting 

 the processes for each in the same way ; and having exposed each for two minutes to 

 a faint daylight, I laid them aside in the dark, to be presently used as test plates, in 

 lieu of the gilded plate g k. 



Then I took three other plates, D, E, F, of the same size, and conducting the pre- 

 paratory processes for each as before, I iodized D in the dark, and mercurialized it 

 forthwith at 170 Fah., 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 mercuri- 

 alized at 170 Fah. 



F was iodized, and exposed to the sun until it began to turn brown, an effect occur- 

 ring almost at once. It was then mercurialized at 170 Fah. 



All these plates then had their sensitive coating removed by hyposulphite, and were 

 thoroughly washed in distilled water and dried. 



574. 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 six- 

 teenth of an inch, or thereabout, by slips of glass. Then I laid them on the level sur- 

 face of the sand-bath, the test plates being kept cool by sponging occasionally with wa- 

 ter. Temperature of the sand, 200 Fah. ; duration of the experiment, fifteen minutes. 



On examination, A, B, C were all found powerfully mercurialized, nor did there seem 

 to be any difference between them. 



575. I consider, therefore, that the shadows, the demitints, the lights, and the solar- 

 ized portions of a Daguerreotype, are covered with mercury ; for at a temperature of 

 200 Fah., 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.* 



576. In (527) I described a remarkable effect which I had noticed in these investi- 

 gations: 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 ap- 

 pearance. The impression thus communicated to the surface, under certain condi- 

 tions, remains there for a long time. During the cold weather last winter, I produced 



* I believe that the most delicate test for the presence of mercury is a slip of silver iodized to a yellow colour, and expo- 

 sed for two or three minutes to a weak daylight. 



