THE CHANGE IS NOT TRANSIENT. 203 



found that the tube in the indigo rises most rapidly, and the others in the order stated 

 before. Therefore we perceive that the same ray commences, carries on, and com- 

 pletes the process. 



923. Few substances can exceed in sensitiveness to light a mixture of chlorine and 

 hydrogen previously tithonized. Brought into the obscure daylight of a gloomy cham- 

 ber, 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 instrument is shaded by a screen. 



924. I have not recorded in this communication a multitude of experiments of de- 

 tail, which go to support the conclusions here drawn, and which will be published at a 

 proper time. 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, which exist most abundantly in the indigo space of the spectrum. That the 

 phenomenon is due to a true absorption, is fully shown in the circumstance, 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 which has been prepared in the dark. 

 A certain space of time elapses before any formation of muriatic acid occurs, during 

 which the absorption in question is going on ; and when that is completed, and the 

 mixture is tithonized, union of the gases begins, and muriatic acid forms. From end 

 to end of the spectrum the action is positive, and differs only in intensity ; but this 

 difference in intensity opens before us new views of the constitution and character of 

 the solar beam. 



University of New- York, June 20, 1843. 



925. The foregoing paper was written almost a year ago, and since that time I have 

 made several new observations corroborative of the results given. 



926. Chlorine is not the only elementary substance in which the rays produce a 

 change. In his chapter on phosphorus, BERZELIUS remarks, " Light produces in it 

 (phosphorus) a peculiar change, of which the intimate nature is unknown ; and which, 

 so far as we can judge at present, does not alter its weight. It makes it take a red 

 tint. This phenomenon occurs not only in a vacuum, even in that of a barometer, but 

 also in nitrogen gas, in carburetted hydrogen, under water, alcohol, oil, and other 

 liquids. When we expose to the sunlight phosphorus dissolved in ether, oil, or hy- 

 drogen gas, it instantly separates under the form of red phosphorus ; it undergoes very 

 rapidly this modification in xiolet light, or in glass vessels of a violet colour. The 

 light of the sun makes it easily enter into fusion in nitrogen gas, but it dors not melt 

 in hydrogen, and in the Torricellian vacuum it sublimes in the form of brilliant red 

 scales." (BERZELIUS, Traite, torn, i., p. 258.) 



