616 PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEIECHES. 
hence 
1 ^ - 
W numlog = ajA 
^~1- 1 H' 
num log = «j^i 
The value of oji is obtained from this equation by a method of approximation. 
Experiments conducted in this manner at 23°-3 C., and under a pressure of 0’7589 
millims., gave the following elements for the calculation of the values of ^ contained in 
the fifth column. Each value of W and 'W^ is the mean of fi-om ten to fifteen obseiwa- 
tions. 
Series of Experiments XI. 
w,. 
W. 
b- 
h. 
1 
“1 
20-97 
19-38 
16-26 
12-34 
4-46 
11-17 
10-26 
6-73 
194-5 
193-8 
114-7 
153-0 
21-5 
65-5 
60-9 
58-6 
millims. 
256 
216 
316 
262 
Mean 262 
Had the insolated chlorine and hydrogen mixture possessed the density corresponding 
to 0° C. and 0‘76 pressure, and if it had been perfectly dry, the length — = 262 would 
have diminished, for we have shown that the amount of light absorbed is proportional to 
the density of the absorbing medium. In a pm-e dry mixture of equal volumes of chlo- 
rine and hydrogen, having a density corresponding to 0° C. and O’ 76 pressure, the length 
of a column of gas through which the light must pass in order to be reduced by optical 
and chemical extinction to -njth of its original amount, is —=234 niilbms. Supposing 
«2 
that the hydrogen were replaced by an equal volume of a chemically inactive gas, which, 
like hydrogen, does not absorb any chemical rays, we should obtain from the coefficient 
of extinction of pure chlorine x^, a length of the gaseous mixture equal to 2x173 or 
346 millims., through which the light would have to pass in order that it should be 
reduced to xoth. Hence it is seen that for a given amount of chemical action effected 
in the mixture of chlorine and hydrogen, an equivalent quantity of light is absorbed. 
For we find that in the standard chlorine and hycbogen mixture, where, together uith 
the thermal effect, a chemical action of the light occurs, the value of the coefficient of 
extinction is 0-00427 ; whereas in a chlorine mixture of similar dilution, where the che- 
mical action was absent, the coefficient was 0-00289, or very much smaller. 
The difference between these two numbers, 0-00138, represents the coefficient of 
extinction due to chemical action alone. The consumption of rays from a coal-gas 
flame, solely employed in the act of photo-chemical combination, is hence represented 
