

Chemical Force in the Spectrum, 439 



graphs as depending for its indications on the union of chlorine 

 and hydrogen, furnishes the means of ascertaining many facts 

 respecting the combination of those substances, since it gives 

 accurate quantitative measures. 



By referring to my papers in the Philosophical Magazine 

 (Dec. 1843, July 1844, Nov. 1845, Nov. 1857), it will be found 

 that chlorine and hydrogen do not unite in the dark at any ordi- 

 nary 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 phenomena present two phases:— 1st. For a brief period 

 there is no recognizable chemical effect, a preliminary actiniza- 

 tion or (as Professors Bunsen and Boscoe subsequently termed 

 it) photochemical induction taking place; it is manifested by 

 an expansion and contraction of the mixture. 2nd. The com- 

 bination of the gases begins, it steadily increases, and soon ac- 

 quires uniformity. In obtaining measures by the use of these 

 gases, we must therefore wait until this preliminary actinization 

 is complete. 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 (Phil. Mag. July 1844). Professors Bunsen 

 and Boscoe, however, in their memoir read before the Boyal 

 Society^ state that it is quite transient (Trans. Boy. Soc. 1856). 

 This preliminary actinization completed, the quantity of hy- 

 drochloric acid produced measures the quantity of the acting- 

 radiation. This I proved by using a gas-flame of standard height, 

 and a measuring-lens consisting of a double convex, 5 inches in 

 diameter, sectors of which could be uncovered by the rotation of 

 pasteboard screens upon its centre, the quantity of hydrochloric 

 acid produced in a given time being proportional to the area of 

 the sector uncovered. The same was also proved by using a 

 standard flame, and exposing the gases during different periods 

 of time. The quantity of hydrochloric acid produced is propor- 

 tional to the time. 



The following experiment illustrates the phenomena arising 

 during the actinization of a mixture of chlorine and hydrogen, 

 and substantiates several of the foregoing statements. 



The diverging rays of a lamp were made parallel by a suitable 

 combination of convex lenses. In the resulting beam a chlor- 

 hydrogen actinometer was placed, there being in front of it a 

 metallic screen so arranged that it could be easily removed or 

 replaced, and thus permit the rays of the lamp to fall on the 

 actinometer or intercept them. 



