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X. " Photochemical Researches." By Prof. BUNSEN and HENRY 

 E. ROSCOE, B.A., Ph.D. 3rd Communication. " The 

 Optical and Chemical Extinction of the Chemical Rays." 

 Communicated by Professor STOKES, Sec. R.S. Received 

 May 20, 1857. 



(Abstract.) 



In order to determine whether the act of photochemical combina- 

 tion necessitates the production of a certain amount of mechanical 

 effect, for which an equivalent quantity of light is expended, or 

 whether this phenomenon is dependent upon a restoration of equili- 

 brium effected without any corresponding equivalent loss of light, we 

 must now study the phenomena occurring at the bounding surfaces, 

 and in the interior of a medium exposed to the chemically active 

 rays. 



If I represents the amount of light entering a medium, and I the 

 amount issuing from the medium, we have aI =I, when a repre- 

 sents the fraction of the original amount of light which passes through 

 the medium, on the supposition that the light extinguished is pro- 

 portional to the original intensity of the light. The first series of 

 experiments was made with the view of determining this point. The 

 intensity of the chemical rays proceeding from a constant source of 

 light was measured before and after passage through a cylinder with 

 plate-glass ends, filled with dry chlorine. The amount of transmitted 

 light, I, was determined for various intensities of incident light, I , and 



the fraction - was found to remain constant, proving that the absorp- 



*o 



tion of the chemical rays varies directly as the intensity of the light. 

 From this result, the general law of the extinction of the optical and 

 chemical rays in transparent media may be deduced. For, as it has 

 been shown that the amount of light transmitted through a medium 

 of finite thickness is proportional to the intensity of the incident light, 

 it may be assumed that this same relation will hold good for an infi- 

 nitely thin medium. According to this supposition, the relation be- 

 tween the transmitted light, I, and the thickness of the medium, is 



represented by the equation I = I .10~* a and a = ^log \^)' * u wmc b 

 I represents the light before transmission, I, that after transmission 

 through a layer of h thickness, and - the thickness of absorbing 



