MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 409 



to a few isolated lines. The gas removes a very large 

 portion of the spectrum. Subsequent experiments must 

 determine whether each of these lines of absorption is 

 also a line of maximum chemical action. 



The chlor- hydrogen actinometer furnishes the means 

 of ascertaining many facts respecting the combination of 

 those substances advantageously, since it gives accurate 

 quantitative measures. 



By referring to my papers in the Philosophical Mag- 

 azine (Dec., 1843; July, 1844; Nov., 1845; Nov., 1857) it 

 will be found that chlorine and hydrogen do not unite 

 in the dark at any ordinary temperature or in any 

 length of time ; but if exposed to a feeble radiation, such 

 as that of a lamp, they are strongly affected. The phe- 

 nomena present two phases: 1st, for a brief period there 

 is no recognizable chemical effect, a preliminary actiniza- 

 tion, or, as Professors Bunsen and Roscoe subsequently 

 termed it, photo - chemical induction, taking place (it is 

 manifested by an expansion and contraction of the mixt- 

 ure) ; 2d, the combination of the gases begins, it steadily 

 increases, and soon acquires uniformity. In obtaining 

 measures by the use of these gases we must, therefore, 

 wait until this preliminary actiuization is completed. 

 That accomplished, the hydrochloric acid arising from 

 the union of the gases is absorbed so quickly that the 

 movements of the index-liquid over the graduated scale 

 give trustworthy indications. 



As regards the duration of the effect produced on the 

 gases by this preliminary actinization, I found that it 

 continued some time several hours (Philosophical Mag- 

 azine, July, 1844), Memoir XIX. Professors Bunsen and 

 Roscoe, however, in their Memoir read before the Royal 

 Society, state that it is quite transient (Transactions 

 of the Royal Society, 1856). 



This preliminary actinization completed, the quantity 



