224 CONDITION OF A DAGUERREOTYPE SURFACE. [MEMOIR XVI. 



Then I took three other plates, D, E, F, of the same 

 size, and conducting the preparatory processes for each 

 as before, 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 dif- 

 fused 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 re- 

 moved by hyposulphite, and were thoroughly washed in 

 distilled water and dried. 



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, separat- 

 ing each pair of plates one sixteenth of an inch, or there- 

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

 surface of the sand-bath, the test-plates being kept cool 

 by sponging occasionally with water. Temperature of 

 the sand, 200 Fahr. ; 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. 



I consider, therefore, that the shadows, the demi-tints, 

 the lights, and the solarized portions of a daguerreotype, 

 are covered with mercury ; for at a temperature of 200 

 Fahr. they all evolve it alike, a sufficiency of vapor rising 

 from the parts that have not been exposed to the light 

 to bring a plate that has been so exposed to its max- 

 imum of whiteness. 



