PHOTOCHEMISTPtT. 



By M . J A M 1 N . 



Translated/or the Smithsonian Institution from the " Revue des Cours scientijiques de la France 

 et de VttruniTp.r.'''' 20 .lulu. lR(i7. 



et de Vetrunger,'" 20 July, 18(57 



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 which 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 i^ 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 capable of possessing three properties — of being- 

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

 other, the raj'S remain identical in their nature, but possessing increasing refran- 

 gibilities and vibratory 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. Tlius, 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 only obscure, 

 that is to sa}', less refrangible, radiations. Thus again, in the curious phenomena 

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

 scarcel\' visible, were absorbed by different bodies, and became transformed finally 

 into luminous radiations. In a word, difierent substances have tlie property of 

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

 selves entering into vibration, and of ^-ielding up by radiation the active force 

 absorbed, w'ith this constant character, that a simple ray has been finally trans- 

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



It is another form of this proposition whicli I am al)Out to expound 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 whicli they have 

 communicated to it, being incapable of being lost, is applied to the production 

 of an equivalent chemical effect. It is with photochemistry, in a word, tluvt 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 I'ays were alone capable of pr(jducing it. He named them, 

 on this account, chemical rays. Wollaston carefully repeated this exj)criment, 

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

 radiations, more refrangible and wholly invisible, but capable of acting ou tho 

 chloride of silver. 



