Prof. Bunsen and Dr. Roscoe^s Photochemical Researches. 231 



found from the formula , , ,~^,. = a . Hence the amount of light 



transmitted by two plates is 0-823. We have confirmed the accuracy of 

 the calculated result by direct experiment, and obtained a value 0*800, 

 or a mean of O'Sll as the coefficient of transmission of our plates. 



If all the transparent media have not the same coefficient of re- 

 flexion, the order in which the media are placed will affect the amount 

 of transmitted light. We have given an example of the mode in 

 which the calculation must in this case be made, in the determination 

 of the coefficient of extinction of water. We found that the amount 

 of light absorbed by a column of water 80 millimetres thick was in- 

 appreciable. According to the method here adopted, it is possible 

 to determine the coefficient of reflexion of all transparent fluids for 

 the chemical rays. We have only determined the coefficient of re- 

 flexion for American mica ; for the chemical rays of a coal-gag flame 

 p was found to be =0-1017. From the coefficient of reflexjQn, the 



refractive index (i) can be calculated from the equation p=^— ^ 



or i= ^-^^^,-. The refractive index for crown glass thus calculated 



l+Vp 

 from p= 0-0509 is found to be i= 1-583; the refractive index for 

 Fraunhofer's line H has been optically determined to be between 

 1-54G6 and 1-5794 (Buff's Physik). 



Another important element in the investigation of photochemical 

 extinction is the law according to which the optical coefficient of 

 extinction varies with the density of the absorbing medium. A series 

 of experiments proved that the amount of chemical rays transmitted 

 through a medium varies proportionally with the density of the ab- 

 sorbing medium. 



We may now proceed to the investigation of the original question 

 proposed, viz. — in the combination of chlorine and hydrogen effected 

 by the light, are the chemical rays expended in a relation proportional 

 to the quantity of hydrochloric acid formed? The first point to be 

 determined, in order to answer this question, is the coefficient of ex- 

 tinction of pure chlorine for the chemical rays of a coal-gas flame. 

 The amount of light was measured before and after transmission 

 through cylinders filled with chlorine. The loss of light by reflexion 

 a=0-811 must be deducted from the incident light, and then the 

 coefficient of extinction for chlorine is calculated. From a series of 

 det^inations, the value of -, i. e. the depth of chlorine at 0°C. and 



O-7G pressure, through which the light must pass in order to be 

 reduced to y'^, is found to be, as a mean of five experiments, 171-7 

 mm. Another series of determinations were made with chlorine di- 

 luted with air, in order to prove experimentally that the absorbed 

 light varies in the case of chlorine directly as the density. The 

 quantity of chlorine contained was determined in each instance by a 

 volumetric analysis. An average of six experiments gave a value for 



1=174-3 mm. As a mean of these two series of experiments we 



a 



have a value of 173-3. 



If the light is not consumed in the act of photochemical change. 



