PROFESSOE BUNSEN AND DR. H. E. EOSCOE’S PHOTO-CHEMICAL RESEARCHES. 017 
by an amount of light which is reduced to by passing through 723 millims. of 
the standard chlorine mixture, whilst in consequence of optical extinction alone the 
light is reduced to by passing through 346 '6 of the same standard mixture. 
It appeared to us of great interest to repeat the experiments with rays from other 
sources of Hght. For this purpose we employed the diffuse light of day reflected from 
the zenith of a cloudless sky. The rays were rendered as nearly parallel as possible by 
reflexion from a mirror through a blackened tube, 6 feet long and 6 centimetres wide, 
on to the insolation-vessel of our instrument. All the observations employed in the two 
next series of experiments (XII. and XIII.), with diffuse light thus reflected, were made 
between the hours of 9 and 12 in the morning. For the determination of the coefficients 
of extinction for pure chlorine, a series of experiments similar to Series VIII. was made. 
Series of Experiments XII., made on the 4th and 6th of August, 1856. 
Exp. 1. 
Exp. 2. 
Exp. 3. 
Exp. 4. 
Exp. 5. 
lo 
15-23 
12-12 
10-84 
9-63 
10-61 
I 
2-96 
2-62 
2-54 
2-51 
2-28 
h 
32-4 mm. 
27-0 
27-0 
27-0 
27-0 
p 
0-7567 
0-7567 
0-7567 
0-7567 
0-7567 
t 
26°-3 
26-3 
26-3 
26-3 
26-3 
The values of the reciprocals of the coefficients of extinction /3, are calculated from 
these numbers according to formula (9.). 
No. of Exp. 
1 
d' 
I 
. . 47-4 
2 
. . 42-8 
3 
. . 45-5 
4 
. . 49-7 
5 
. . 42-5 
Mean 
. . 45-6 
The coefficient of optical extinction of pure chlorine, at a density corresponding to 0° C. 
and 0’76 pressure, for diffase morning light reflected from the zenith, is hence 0’02I9, 
very different from the value 0-00578 found for a coal-gas flame. This result renders 
it probable that the coefficient of extinction of the standard chlorine and hydrogen 
mixture is much greater for diffuse morning light than for light from a coal-gas flame. 
The following experiments made with the chlorine and hydrogen mixture at 23°-8 C. and 
0-7535 Barom., -with diffuse zenith light, also between the hours of 8 a.m. and 12, gave 
results resembling Series XI. 
