98 British Association for the Adva7icement of Science. 



of Optics and Chemistry united, were insufficient in their present 

 state, to give any plausible explanation of this delicate and com- 

 plicated process. If M. Arago, who had the advantage of being 

 for six months acquainted with the secret, and therefore of con- 

 sidering its nature in all points of view, was of this opinion, it 

 seemed as if a call were made on all the cultivators of science to 

 use their united endeavors, by the accumulation of new facts and 

 arguments, to penetrate into the real nature of these mysterious 

 phenomena. For this reason he would offer a small contribution 

 of new observations, which might perhaps be of service in the 

 elucidation of this new branch of science. The first part of 

 Daguerre's process consists in exposing a silver plate to the vapor 

 of iodine, by which it becomes covered with a stratum of iodide 

 of silver, which is sensitive to light. Mr. T. stated that this fact 

 had been known to him for some time, and that it formed the 

 basis of one of the most curious of optical phenomena, which as 

 it did not appear to have been observed by Daguerre, he would 

 here describe. Place a particle of iodine, of the size of a pin's 

 head, on a plate of silver, or on a piece of silver leaf spread on 

 glass. Warm it gently, and you will shortly see the particle sur- 

 rounded with colored rings, whose tints resemble those of New- 

 ton's rings. Now, if these colored rings are brought into the light, 

 a most singular phenomenon occurs ; for the rings prove to be sen- 

 sitive to the light, and their colors change, and in a short time 

 their original appearance is quite gone, and a new set of colors 

 occupy their places. These new colors are altogether unusual 

 ones; they do not resemble any thing in Newton's scale, but 

 seem to have a system of their own. For instance, the two first 

 colors are deep olive green, and deep blue inclining to black, 

 which is quite unlike the commencement in Newton's scale. It 

 will be understood that the outermost ring is here accounted the 

 first, being due to the thinnest stratum of iodide of silver, farthest 

 from the central particle. The number of rings visible is some- 

 times considerable. In the centre of all, the silver leaf becomes 

 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 colored rings seem to consist of iodide of silver in 

 various stages of development. They have a further singular 

 property, which, however, has not been sufficiently examined 



