PHOTOGRAPHY. 5 



silver-haloid. It should also be noted that however short 

 the exposure to light may be, the same changes occur in a 

 greater or less number of the molecules though such changes 

 may be invisible to the eye, owing to the preponderance of 

 the unaltered salts. 



Now when Daguerre's plates were exposed in the camera 

 for a short time, no visible image was apparent, but, never- 

 theless, some minute quantity of the Ag 2 T 2 was converted 

 into Ag 2 T. It was found by Daguerre that such an invisible 

 image had the power of condensing mercury from mercury 

 vapour on the parts forming it, and that metallic lustre was 

 given to it. Thus the Daguerrean image of this white piece 

 of paper would have been represented by mercury and 

 sub-iodide of silver, while this black piece of paper would 

 have been represented by the silver iodide ; and when the 

 unaltered iodide is dissolved away, the latter would be re- 

 presented by the dark-coloured silver, and the former by the 

 lighter amalgam of silver and mercury. 



I have here a glass plate silvered by Liebig's process, and 

 we will place it for a couple of minutes in this common 

 deal box, 1 at the bottom of which is a piece of cardboard 

 which has been exposed to iodine vapour during the night. 

 Iodine volatilizes at ordinary temperatures, so by leaving it 

 in the box the surface will be converted into silver iodide 

 by the combination between the metal and the halogen. I 

 now withdraw it, and examining it by candle light, 1 find it 

 of a delicate canary colour, with a slight tint of rose. It is 

 now in a sensitive condition to ordinary light. I will not 

 waste your time by exposing the plate in the camera, but 

 will place it behind a glass negative picture (what that 

 means we shall learn presently), and expose it for a few 

 seconds in the beam of the electric light. I believe it is 

 sufficiently exposed, so I will take it and hold it in the 

 vapour coming from the mercury (heated over a Bunsen 

 burner to about 150 F.) which is in this small capsule. The 

 image begins to spring out at once, and after a little longer 

 treatment it is fully developed. [The picture was handed 

 round.] 



Now silver iodide, as I said, is sensitive to light, that is, 

 light changes it from the iodide to the sub-iodide so long as 



1 The plate was supported on a couple of deal strips laid on the 

 card. 



