632 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



ested in the facts now presented. However, it behoves all bitu- 

 minous coal miners and dealers, and more especially coal-gas com- 

 panies to protect, as far as possible, their coal from exposure, and 

 never to gather it in large heaps while it is wet. 



Action of Light on Iodine of Silver. 



At the November meeting of the American Photographical 

 Society, Professor S. D. Tilhnan remarked that the subiodide of 

 silver is not necessarily formed by the action of light on the 

 iodine. M. Carey Lea, in an able paper, to be found in the Sep- 

 tember number of SilUman's Journal, "on the nature of the 

 action of light upon iodide of siliv'er," has very clearly shown 

 that pure isolated iodide of silvc^r undergoes no reduction when 

 exposed to light many thousand tim.es longer than it usually is in 

 the photographic process. The original amount of silver and of 

 iodine remains after such exposure. Hence, he concludes there is 

 no chemical action. What the nature of the change is we do not 

 yet comprehend. It may be that a new class of metamers are 

 formed, which are not to be distinguished by different atomic 

 arrangement. We can conceive that the atom mav undergo a 

 metamorphism as the result of a modification of its normal motion, 

 and on the withdrawal of the exciting cause, it may resume its 

 original condition. There is a large class of metallic salts which, 

 at a low temperature, absorb from the air an atom of oxygen, and 

 at a higher temperature, part with it. Alternating action may be 

 thus continued for any length of time by a change of tempera- 

 ture. This is a true chemical action. In the case of the eflect of 

 light upon the silver haloid, the action may be chemical, although 

 there may be no change in the amount of material. 



We assume in cases of isomerism that there is a change in the 

 arrangement of atoms. For instance, we give to every one of the 

 nine isomers of Rutic acid a different formula, while, in fact, each 

 consists of ten atoms of carbon, twent}'^ of hydrogen, and two of 

 oxygen, according to the new notation. It cannot be proved that 

 these different arrangements of atoms actual I}'- exit ; yet these 

 fo] multe materially assist us in conceiving how different chemical 

 functions may belong t® bodies of the same ultimate composition. 



Again, we may suppose such chemical action takes place as to 

 produce homologues, which arc simple multiples of the atom of 

 iodide of silver. Light not only decomposes carbonic anhydride, 

 by means of the leaf, but it probabl}- so modifies the atoms of 



