428 Dr. Waller's Experiments on the 



face of silver or copper, and in a short time we find around 

 the iodine a series of coloured zones of the various tints of 

 the spectrum, and approaching in a greater or less degree to 

 the form of a circle, according as they have been more or less 

 disturbed in their formation by currents of the surrounding 

 air. In order that they may be perfectly regular, as large as 

 possible, and with tints undisturbed by the action of light, it 

 is necessary to place a piece of iodine in the centre of a well- 

 polished plate, as before described ; this is then to be shaded 

 by an opake screen superimposed a few lines from the surface 

 to cause the vapours which would otherwise ascend and par- 

 tially escape, to expand over its silver surface. Coloured 

 rings may be formed in the same manner by bromine and 

 chlorine and the various combinations of these bodies with 

 each other, except that for those that are gaseous or liquid it 

 is requisite to pay a little attention to the manner of disen- 

 gaging them on the surface of the metal, either by passing 

 them through a glass tube, or by some other contrivance easy 

 to execute. These rings correspond to those formed by re- 

 flected light in Newton's experiments, with this difference, 

 however, that in the coloured films of the soap bubble, and 

 in those formed by the glass lenses, the thinnest film is in the 

 centre ; whilst in these rings, obtained by chemical action, it 

 exists at the circumference, as is the case with the coloured 

 rings of Nobili. In watching the formation of these pheno- 

 mena, at first are seen two or three very small circles, {which 

 appear almost as soon as the iodine and the metal are placed in 

 contact with each other ; as the experiment continues, the cir- 

 cumferences of these circles become gradually greater ; whilst 

 the external colours extend themselves over a greater space, 

 those of the centre grow fainter ; red and green now only re- 

 main visible, and these at last, when the film has attained a 

 certain thickness, in their turn also give place to a dull coating 

 of brown. The formation of these rings evidently depends 

 on the vaporization of the iodine from the solid nucleus. The 

 variety in colour and extent of these zones is caused by the 

 difference between the strength of the vapour at the centre 

 and the circumference of the iodic atmosphere whilst expand- 

 ing over so large a surface. In the metal thus combining with 

 the vapour, we have to consider, — 1, the force of the vapour 

 at different distances from the centre ; 2, the obstacle which 

 a film of iodine, once formed, opposes to any further action 

 between the iodine and the metal. 



This experiment may be varied in different ways: two 

 pieces of iodine of about the same size, placed at a small di- 

 stance from each other on a silver plate, form separate co- 



