PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 369 
In order to eliminate the more serious errors arising from the radiant heat of the source 
of hght, we placed, as we previously mentioned, a glass cylinder, N, Plate XVII. fig. 2, 
closed by plates of glass and filled with water, between the source of light and the inso- 
lation-vessel In addition to this water-cylinder, the insolation-vessel was further pro- 
tected fi’om access of radiant heat by the double metalhc screen, fig. 4, placed between 
the insolation-vessel and the water-cylinder N, fig. 2. The apertures in the middle of 
each sheet of metal were closed by perfectly clear thin plates of mica, behind which was 
placed the insolation-vessel ^ contained in the blackened cap c and covered by the lid d. 
Lateral slits in the cap, with others corresponding in the lid, allowed the tubes oo to 
pass out, and currents of air were excluded by stopping the lateral slits with black wool. 
As it is important to observe the height of the flame whilst the absorption is being read 
otf on the scale, a small hole, I, was made in the lid d, through which the point of the 
flame could be easily seen. 
By this arrangement all errors arising from radiant heat were completely excluded, 
for it was found that if the vessel i were filled vdth a gas which is not sensitive to light, 
the stand of the water in the observation-tube remained constant, whether the gas were 
exposed to the hght or not. 
It now only remained to determine the efiect produced on the indications by the heat 
evolved from the combination of the gases. A phenomenon of great importance here pre- 
sented itself. If the insolation-vessel be suddenly darkened during the normal working 
of the apparatus, it is found that the action does not instantly cease, but that the volume 
Sp of gas read ofi" at the moment of darkening diminishes after the lapse of the times 
the volumes so that for the times — &c., after the light 
had been cut off, a corresponding decrease of volume (^p— Si)(Si— Sj) is found. The time 
during which this decrease of volume takes place is a very short one, as may be seen 
from the following experiments, made at 20°’9 C. and 0“‘7497 Barom. 
Series of Experiments VII. 
Experiment 1. 
Experiment 2. 
Experiment 3. 
Time in 
seconds. 
t. 
Observa- 
tions on 
the scale.* 
s. 
/i — /p. 
«p — Sl. 
Time in 
seconds. 
t. 
Observa- 
tions on 
the scale. 
s. 
— ^0- 
Sp — *2. 
Time in 
seconds. 
t. 
Observa- 
tions on 
the scale. 
s. 
ti — tg. 
Sp — *2. 
Insolated 
Darkened 
U 
0 
60 
120 
180 
190 
200 
210 
220 
230 
240 
241-0 
253-5 
265-5 
278-0 
279-5 
279-7 
279-8 
279-8 
279-8 
279-8 
60 
60 
60 
10 
10 
10 
10 
10 
10 
12-5 
12-0 
12-5 
1-5 
0-2 
0-1 
0-0 
0-0 
0-0 
Insolated 
Darkened 
H 
0 
60 
120 
130 
140 
150 
160 
170 
293-5 
307-5 
321-5 
323-0 
323-3 
323-4 
323-4 
323-4 
// 
60 
60 
10 
10 
10 
10 
10 
14-0 
14-0 
1-5 
0-3 
0-1 
0-0 
0-0 
Insolated 
Darkened 
// 
0 
30 
60 
90 
120 
150 
339-5 
322-0 
306-0 
301-0 
300-0 
300-0 
tt 
30 
30 
30 
30 
30 
17-5 
16-0 
5-0 
1-0 
0-0 
• The increasing numbers on the divided arbitrary scale of the instrument correspond to a diminution of 
the volume of the total gas. 
MDCCCLVII. 3 C 
