OF ARTS AND SCIENCES. 101 



The oxi-iodides of antimony, being insoluble in carbonic disnlphide, 

 are precipitated as an amorphous yellow powder, while the free iodine 

 remaining in the solution changes its original greenish yellow color to 

 a deep red, so deep that it soon appears black by reflected light. The 

 change of color in the direct sunlight is very rapid, and forms a most 

 striking phenomenon, which can readily be shown on the lecture table. 

 When the direct rays of the sun are not available, the reaction can be 

 produced by burning a few feet of magnesium ribbon. It is by far the 

 most striking example of oxidation by the sun's light which has yet 

 been discovered ; and may, therefore, as a lecture experiment, be 

 brought in striking contrast with the reduction of argentic chloride 

 by the same agent, — a change which it rivals in extent, if not in 

 rapidity. It has been maintained * that while the more refrangible 

 rays of the solar spectrum exert a reducing action on metallic com- 

 pounds, both binaries and salts, the less refrangible rays (the yellow 

 as well as the red) produce the contrary effect, and tend to increase 

 the oxidizing action of the atmosphere. In the phenomenon we are 

 studying, the oxidation is actually determined by the sun's light, and 

 in the most marked manner ; and, as this is the first definite example 

 of such action which has been observed, it became a very interesting 

 question to inquire, in what part of the solar spectrum the action was 

 most intense. We therefore exposed the solution, in test-tubes, to 

 the sun's rays at different parts of the solar spectrum, but under other- 

 wise identical conditions ; taking care, of course, to protect the tubes 

 from any other radiation. The spectrum was projected in the usual 

 way, with a lense and prism of glass ; and we found that, while the 

 brilliant red and yellow rays caused no sensible change of color, the 

 comparatively faint blue and violet rays produced a very marked 

 effect. Our method of experimenting was not delicate enough to 

 show the precise point of maximum eff"ect; but it was evident that the 

 whole order of the phenomena was the same as in the case of argentic 

 chloride and similar photographic preparations. 



As we have before stated, the solution of iodide of antimony under- 

 goes no change in contact with ordinary air, so long as it is kept in the 

 dark ; and since the amount of iodine set free under the influence of 

 the light can be readily estimated, and since this is the measure of the 

 chemical action, it is evident that the new reaction affords a direct 



* ^fetude sur la Part de la Lumiere dans les Actions Chimiques, et en particu- 

 lier dans les Oxydations par M. P. Chastaing. Annales de Cliimie et de Phys- 

 ique (5). XI. 145. June, 1877. 



