382 PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAX EESEAECHES. 
The fact that the action of light upon a mixture of chlorine and hydrogen does not 
take place instantaneously, but that a considerable continuation of the exposure is neces- 
sary before an action occurs, was noticed by Deapee in 1843. From this observation he 
concluded that chlorine, on exposure to light, undergoes an allotropic modification, per- 
manent for several weeks, which differs from ordinary chlorine by possessing a greater 
combining power. 
We shall show that the true cause of this phenomenon is totally a difierent one ; that 
it cannot be explained by the supposition of an allotropic modification of chlorine, but 
that the peculiar action which we have called photo-chemical induction here comes into 
play. 
If the chemical rays emanating from a constant source of light be allowed to fall upon 
a mixture of chlorine and hydrogen which has stood for some time in the dark, no ap- 
preciable quantity of hydrochloric acid is formed dm’ing the first moments. After some 
time has elapsed, a small action is observed which very gradually increases, until after a 
considerable space of time a permanently constant maximum action is obtained. 
The following experiments give an example of this gradual increase of the action by 
constant insolation. The first vertical division contains the times of insolation in minutes ; 
the others give the volumes of hydrochloric acid formed by exposiu’e during the times 
contained in the first division. In Experiments 3, 4, 5 and 6, lamplight was employed ; 
in Experiments 1 and 2 diffuse sunlight from the zenith was used. 
Series of Experiments I. 
I . 
DiiFuse sunlight. 
Lamplight. 
II . 
III . 
IV . 
V . 
VI . 
tui. 
Time in 
minutes. 
Exp. 1. 
Exp. 2, 
Exp. 3. 
Exp. 4. 
Exp. 5 . 
Exp. 6. 
1 
0-0 
0-0 
0-0 
0-0 
6-25 
4-13 
2 
0-0 
0 0 
1-58 
0-0 
4-71 
10-42 
3 
0-9 
0-0 
0-50 
00 
6-26 
10-42 
4 
1-0 
0-21 
0-00 
0-0 
5-20 
13-53 
5 
1-3 
0-60 
0-50 
0-0 
5-21 
11-07 
6 
2-0 
1*01 
2*09 
1-04 
10-43 
10-83 
7 
2-2 
0-41 
14-58 
3-13 
11-46 
15-63 
8 
1-7 
1-61 
29*18 
19-78 
10-42 
14-37 
9 
3-0 
1-70 
31 - 11 * 
29-19 
13-33 
20-24 
10 
5-2 
2*73 
30-36 
32 - 68 * 
16-20 
25-30 
11 
5 - 8 * 
3-43 
32-38 
33*40 
20*24 
26-44 
12 
5-7 
6-34 
25-30 
45-74 
13 
7 - 67 * 
32-98 
53-00 
14 
38-00 
53-00 
15 
47 * 00 * 
55 - 00 * 
16 
47-0 
55-00 
17 
46-0 
55-00 
18 
47-0 
* In this and aU the following experiments, the point at which the maximum action is reached is denoted 
by an asterisk. The mean of the numbers below this asterisk is the maximum action for the particular 
experiment. 
