330 Royal Society : — 



" Photocliemical Researches." By Prof. Bunsen and Henry E. 

 Roscoe, B.A., Ph.D. 3rd Communication. "The Optical and 

 Chemical Extinction of the Chemical Rays." 



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

 amount issuing from the medium, we have al,|=l, when a repre- 

 sents the fraction of the original amount of hght 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- 



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"'''"anda=^log {jj, in which 



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



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



medium by passing through which the amount of light has diminished 



tO-jlyth. 



The difference between the incident and transmitted light, i. e. that 

 lost in passing through the medium, is made up (I) of a portion re- 

 flected, and (2) of a portion absorbed or extinguished. We hare ex- 

 perimentally determined the values of the coefficient of reflexion ^, 

 and the coefficient of extinction a, for the glass plates used in our 

 cylinders. We found that 4-8G per cent, of the chemical rays, from 

 a flame of coal-gas, which fall perpendicularly on a surface of crown 

 glass, are lost by the first reflexion ; and that the amount of light 

 absorbed in our plates was so small as to fall within the limits of 

 observational errors. The value of p for]the plates of glass employed 

 was found to be 0'0509. When the coeflScient of reflexion for 

 glass p is known, the amount of light a transmitted by n plates is 



