PHOTOCHEMISTRY. 



B V M. J A M I N . 



Translated/or the Smithsonian Institution from the " Rfivue des Cours scientifiques de la France 

 et de Vetranger" 20 July, 1867. 



Solar radiations, and in general those of all luminous bodies, are composed 

 of a multitude of superposed vibrations endowed with very distinct properties, 

 and which it is practicable to isolate by means of the prism. The first, and 

 least refrangible, are obscure, and manifest themselves to our organs only by 

 the calorific phenomena which they produce. These are followed by the lumin- 

 ous radiations whicli succeed one another from the extreme red to the ray H 

 of the spectrum. To the violet rays, finally, succeeds a large number of radi- 

 ations, invisible it is true, but whose existence is revealed by their power of 

 effecting decompositions or chemical combinations. 



I have heretofore insisted on the point that there were not three special classes 

 of radiations superposed at a given point of the spectrum, and differing by their 

 nature itself, but a single ray capalde of possessing three properties — of being 

 calorific, luminous, and actinometric. From one end of the spectrum to the 

 other, the rays remain identical in tlieir nature, but possessing increasing refran- 

 gibilities and vil)ratory velocities, more and more rapid. I have shown, by the 

 phenomena of phosphorescence and fluorescence, that these different rays can 

 be transformed one into the other. Thus, when luminous radiations, simple 

 and well defined, fall on a metallic plate, they are at first absorbed by degrees ; 

 the plate grows warm; it then radiates in its turn, but gives out onh' obscure, 

 that is to say, less refrangible, radiations. Thus again, in the curious phenomena 

 of phosphorescence and fluorescence, we have seen that the ultra violet radiations, 

 scarcely visible, were absorbed by different bodies, and became transformed finally 

 into luminous radiations. In a word, different substances have the proi)erty of 

 selecting and absorbing certain simple radiations in preference to others, of them- 

 selves entering into vibration, and of 3'ielding up by radiation the active force 

 absorbed, with this constant character, that a simple ray has l)een finally trans- 

 formed into an assemblage of other mixed radiations, all less refrangible. 



It is another form of this proposition which I am about to exi)ound in the 

 course of this lecture. I propose to show that the medium may retain the 

 vibrations which it has absorbed, and that the active force which they have 

 conununicated to it, being incapable of being lost, is applied to the ])roduction 

 of an equivalent chemical effect. It is with photochemistry, in a word, tluat we 

 shall now occupy ourselves. 



Scheele discovered, in 1770, that chloride of silver exposed to the light 

 assumes a violet tint, but he did not stop there. Proceeding to inquire to what 

 simple radiations this phenomenon was due, he studied it in the spectrum, and 

 found that the violet rays were alone capable of jjnxlucing it. He named them, 

 on this account, dmnical rays. Wollaston carefully repeated this exiK-riment, 

 and observed that there existed beyond the ultra violet radiations still other 

 radiations, more refrangible and Avholly invisible, but capable of acting on the 

 chloride of silver. 



