518 



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 



determinations, the value of -, i. e. the depth of chlorine at C. and 



0'76 pressure, through which the light must pass in order to be 

 reduced to -j^-, 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 



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



CL 



have a value of 173'3. 



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

 the coefficient just found must remain unaltered when the chlorine 

 and hydrogen mixture is employed ; if on the contrary light is not 

 only lost by the optical extinction, but an amount of light vanishes 

 proportional to the chemical action, experiment must give a larger 

 value for the coefficient. 



In order to determine this important question we employed an 

 apparatus (fully described in the Paper), by means of which we 

 could expose columns of the sensitive gas of various lengths, to 

 a constant source of light. By determining the amount of action 

 effected in these columns of varying length, we are able to obtain the 

 value of the coefficient of extinction for the sensitive mixture. A 

 series of experiments showed that when the light had passed through 



