4 REPORT — 1839. 



white and semi-transparent, like ivory. This white spot, when heated, 

 turns yellow, again recovering its whiteness when cold : from which it 

 is inferred to consist of iodide of silver in a perfect state. The coloured 

 rings seem to consist of iodide of silver in various stages of develop- 

 ment. They have a further singular property, which, however, has not 

 been sufficiently examined into. It is as follows : It is well known that 

 gold-leaf is transparent, transmitting a bluish green light ; but no other 

 metal has been described as possessing coloured transparency. These 

 rings of iodide of silver, however, possess it, being slightly transparent, 

 and transmitting light of different colours. In order to see this, a small 

 portion of the film should be isolated, which is best done by viewing 

 it through a microscope. Mr. Talbot said, that he had considered the 

 possibility of applying a silver plate thus combined with iodine to the 

 purpose of photogenic drawing, but he had laid it aside as insufficient 

 for that purpose, on account of its sensitiveness appearing to be much 

 inferior to that of paper spread with chloride of silver, and therefore in 

 an equal time it takes a much feebler impression. Now, however, M. 

 Daguerre has disclosed the remarkable fact, that this feeble impression 

 can be increased, brought out, and strengthened, at a subsequent time, 

 by exposing the plate to the vapour of mercury. Another experiment 

 was then related, in which a particle of iodine was caused to diffuse its 

 vapour over a surface of mercury. In order to this, a copper plate was 

 spread over with nitrate of mercury, and then rubbed very bright, and 

 placed in a closed box along with a small cup containing iodine. The 

 result was, a formation of Newton's rings of the greatest splendour and 

 of a large size. But they did not appear to be in any degree sensitive 

 to light. 



The next point of M. Daguerre's process is, the exposure of the 

 picture to the vapour of mercury ; and this is by far the most enig- 

 matical part of the whole process. For, he states that if you wish to 

 view the picture in the usual manner, that is, vertically, you must hold 

 the plate inclined to the vapour at an angle of 45", and vice versa. 

 Now this is something altogether extraordinary ; for who ever heard of 

 masses of vapour possessing determinate sides, so as to be capable of 

 being presented to an object at a given angle ? From the hasty consi- 

 deration which he had been able as yet to give to it, his first impression 

 was, that this fact bore a certain analogy to some others which he 

 would mention. If a piece of silver-leaf is exposed to the vapour of 

 iodine, however uniform the tension of the vapour may be, yet it does 

 not combine uniformly with the metal, but the combination commences 

 at the edge of the leaf and spreads inwards, as is manifested by the 

 formation of successive bands of colour parallel to the edge. This is 

 not peculiar to silver and iodine, but occurs when other metals are ex- 

 posed to other vapours : not always with entire regularity, but there is 

 a tendency to combine in that way. A possible explanation is, that 

 this is due to the powerful electrical effect which the sharp edges and 

 points of bodies are known to possess ; in fact, that electricity is either 

 the cause or the attending consequence of the combination of vapour 

 with a metallic body. Again, if a minute particle of iodine is laid on 



