618 PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEinCAL EESEAECHES. 
Series of Experiments XIII., made on tlie 3rd, 6th and 7th of August, 1856. 
No. of 
'Exper. 
Wi. 
W. 
h,. 
h. 
1 
/>i’ 
1. 
10-31 
5-79 
125-5 
29-0 
millims. 
85-5 
2. 
12-51 
6-03 
132-5 
23-9 
87-0 
3. 
10-85 
7-31 
176-9 
40-9 
85-4 
4. 
8-32 
5-43 
187-3 
33-8 
74-1 
Mean ... 
83-0 
The coefficient of extinction for morning zenith light in a dry mixture of chlorine and 
hydrogen, whose density corresponds to 0°C. and 0’76 pressure, is found to be =0'0136, 
and its reciprocal =73*5 millims. The value of the coefficient of the light consumed in 
effecting the change is =0-00265, and its reciprocal 337-3 millims. From this it is seen 
that the light reflected in the morning from the zenith of a cloudless sky, is much more 
easily extinguished by passing through chlorine than the rays from a coal-gas flame. 
Hence it was conjectured that probably the solar light might difier in its properties 
according to the time of day or year. This supposition has been fully confirmed, as is 
shown by the following experiments made on the 15th of September, 1856, between the 
hours of 3 p.M. and p.m., with zenith hght from a cloudless sky. 
Series of Experiments XIV. 
Exp. 1. 
Exp. 2. 
I„ 
4-81 
6-86 
I 
1-01 
1-60 
h 
11*8 
11-8 
P 
0-7597 m. 
0-7597 
t 
17°'5 
17^-5 
These observations give the following numbers for the reciprocal of the coefficient 
No. of Exp. —• 
y 
1 18-9 millims. 
2 20-5 miUims. 
1 
r 
Mean . . 19-7 
The coefficient of extinction of dry chlorine at 0°C. and 0-76 pressure for the evening 
hght reflected from a cloudless zenith, hence is 0-05076. The optical extinction of 
reflected cloudless zenith light for chlorine at 0° C. and 0-76 pressm-e, was more than 
twice as small on the 4th and 6th of August in the morning, than it was on the 15th of 
September in the evening. A perfectly analogous difference is observed between the 
former series of Experiments XIII. and the following, made with the standai-d mixture, 
at 25°-7 C. and 0-7565 millim. pressure, on the 2nd of August, 1856, between the 
hours of 3^ and 5 p.m., with light reflected from the zenith of a cloudless sky. 
