MEMOIR XIX.] MODIFIED CHLORINE. 283 



periods: the first during which a simple absorption is 

 taking place without a chemical effect, the second during 

 which absorption is attended with the production of 

 hydrochloric acid. 



The facts which I am endeavoring to set forth promi- 

 nently in this Memoir are 1st, the preliminary modifi- 

 cations just discussed; and, 2d, the persistent character 

 of the change impressed upon chlorine when it has been 

 exposed to the sun, an effect wholly unlike a calorific 

 effect, which would soon disappear. 



By resorting to the chlor-hydrogen photometer we ob- 

 tain information equally distinct upon the second, point, 

 that the preliminary modification is not a transient effect 

 which at once passes away, but is, on the contrary, a per- 

 sistent change. 



I modified the chlorine and hydrogen contained in the 

 instrument, and kept it in the dark for ten hours. On 

 exposure to the lamp-rays it moved after a few seconds, 

 showing, therefore, that the change impressed on the 

 chlorine was not lost. In the former case 600 seconds 

 had elapsed before any movement was visible. 



When, however, we remember that the invisible im- 

 ages on daguerreotype plates, and even photographic im- 

 pressions on surfaces of resin, and probably all other 

 similar changes, are slowly effaced, it would be prema- 

 ture to conclude that modified chlorine does not. revert 

 to its original condition. I have sometimes thought 

 that there were in several of niy experiments indications 

 that this was taking place, but would not be understood 

 as asserting it positively. Whether it be so or not, one 

 thing is certain, that the taking on of this condition and 

 the loss of it is a very different affair from any transient 

 exaltation of action due to a temporary elevation of tem- 

 perature, or the contrary effect produced by cooling. 



April 26, 1844. 



