296 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX. 



between the fixed line i and the violet termination, and 

 is probably affected by all those waves the lengths of 

 which are between 0.00001587 and 0.00001287 of a 

 Paris inch ; and inasmuch as it absorbs luminous rays 

 included between the same limits, it is to this absorption 

 that its yellow color is due. 



In these Memoirs the same result is established by me 

 in another way. I found that a ray which had passed 

 through a given thickness of a mixture of equal volumes 

 of chlorine and hydrogen lost by absorption just half as 

 much of its original intensity as when it passed through 

 the same thickness of pure chlorine gas ; a result which 

 obviously leads to the conclusion that when chlorine 

 and hydrogen unite under the influence of the sun, they 

 discharge different functions the chlorine an active, and 

 the hydrogen a passive function. The primary action 

 or disturbance takes place upon the chlorine, and a dis- 

 position is communicated to it enabling it to unite read- 

 ily with the hydrogen. 



By arranging in the spectrum a series of tubes con- 

 taining a mixture of these gases, it was found that 

 the gases placed in the indigo space went into union 

 first. 



These various experiments enabling me thus to trace 

 to the chlorine the source of disturbance, I have next to 

 remark that chlorine which has been exposed to the rays 

 of the sun has gained thereby a tendency to unite with 

 hydrogen not possessed by chlorine made and kept in 

 the dark. In proof of this fact I may cite an experiment 

 from Memoir XIX. : 



" In two similar glass tubes place equal volumes of 

 chlorine made from peroxide of manganese and hydro- 

 chloric acid by lamplight, and carefully screened from 

 access of daylight Expose one of the tubes to the full 

 sunbeams for some minutes, or, if the light be feeble, for a 



