PEOPESSOE BTJNSEN AOT) DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 359 
rine and hydrogen take place in the dark at ordinary atmospheric temperatures, or is 
the presence of hght a necessary condition'? For the purpose of determining this ques- 
tion, the experiments just cited, calculated according to formula (1.), were repeated with 
the difference, that the analyses were made after the vessels containing the gaseous 
mixtures had remained for some hours in the dark. We obtained, — 
Series of Experiments II. 
No. 
Capacity 
of vessel. 
Cl- 
Tempe- 
rature. 
T. 
Barometer. 
P. 
W. 
t. 
t'. 
a. 
Chlorine 
found. 
c. 
Chlorine 
calculated. 
iCj. 
21 hours in dark... < 
2 hours in dark... ■ 
' 
I. 
II. 
III. 
IV. 
V. 
190-24 
113-03 
73-42 
51-60 
61-01 
O 
11-1 
11-1 
11-1 
12-9 
12-9 
0-7265 
0-7265 
0-7265 
0-7393 
0-7393 
5 
3 
2 
2 
2 
92-0 
89-9 
39-8 
76-7 
71*9 
59-9 
34-4 
26-0 
45-7 
15-2 
0-0024869 
0-0024869 
0-0024869 
0-0024864 
0-0024864 
87-57 
51-50 
33-62 
23-57 
28-14 
87-38 
51-91 
33-72 
23-96 
28-34 
These experiments show, that, after standing from two to twenty-one hours in the dark, 
only from I to 1*5 per cent, of the total chlorine had disappeared. The numbers in the 
two last vertical columns prove clearly that the electrolytic chlorine and hydrogen mixture 
remains unaltered when light is excluded, and that if the small differences which occur 
do not arise altogether from errors of observation, still then- influence on the photo- 
chemical determinations is so small that it may be enthely neglected. 
Having thus assured ourselves, in the foregoing experiments, that the electrolysis of 
aqueous hydrochloric acid affords a simple and rehable method of preparing a gaseous 
mixtui’e of perfectly constant composition, which is unalterable in the dark, but sensitive 
when exposed to hght, it now only remained carefully to examine all the circumstances 
which could possibly modify the action of light on the mixture, so that in the determi- 
nations they may be either enthely removed or allowed for in the calculations. 
As we do not deem it necessary here to enumerate the various preliminary experi- 
ments which at last led us to the wished-for result, but tried our patience for upwards 
of half a year, we proceed at once to the description of the instrument by means of 
which we have been ultimately enabled to exclude aU distm'bing causes from our deter- 
minations, and have succeeded in referring the chemical action of light, not only to a com- 
parative, but, as we shall show in a later section, even to an absolute measurement. 
The arrangement of the apparatus we have employed is as follows : — The glass tube, a, 
fig. 2, blown before the glass blowpipe, serves for the electrolysis of the hydrochloric acid, 
and contains two electrodes of carbon, which are connected with three or four common- 
sized zinc-carbon elements, C, by means of the platinum wires hb melted through the 
glass. Between this vessel a and the battery C, is placed the gyro trope D, by means 
of which the resistance in the circuit can be at any time greatly increased ; this is 
instantly effected by reversing the balance-wire, so that the current passes through the 
smaU decomposing cell d containing a very dilute acid. By including and excluding 
