280 MODIFIED CHLOKINE. [MEMOIR XIX. 



it is remarkable how promptly the level of the liquid in 

 the tube rises; how, when the shutters are successively 

 thrown open, the action becomes more and more ener- 

 getic ; and how, in an instant, it stops when the instru- 

 ment is shaded by a screen. 



I have not recorded in this communication a multi- 

 tude of experiments of detail supporting the conclusions 

 here drawn. It has been my object on this occasion to 

 call attention to the fact that chlorine, an elementary 

 body, undergoes a change after exposure to the light ; a 

 change which appears to produce an exaltation of its 

 electro-negative properties, as is shown by its power of 

 uniting more energetically with hydrogen. This change 

 must not be confounded with those transient elevations 

 of activity due to increased temperature, inasmuch as 

 this is more permanent in its character. It arises from 

 the absorption of rays existing most abundantly in the 

 indigo space of the spectrum. That the phenomenon is 

 due to a true absorption is fully shown by the circum- 

 stance that a beam which has produced this effect has 

 lost the quality of ever after producing a similar result. 

 This is borne out by what we observe to take place 

 when a feeble light falls on a mixture of chlorine and 

 hydrogen prepared in the dark. A certain space of 

 time elapses before any formation of hydrochloric acid 

 occurs, during which the absorption in question is going 

 on ; and when that is completed, and the mixture is 

 modified, union of the gases begins and hydrochloric 

 acid forms. From end to end of the spectrum the action 

 is positive, and differs only in intensity ; but this differ- 

 ence in intensity opens before us new views of the con- 

 stitution and character of the solar beam, 



UNIVERSITY OF NEW YORK, June 20, 1813. 



