PEOFESSOE BTJNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 303 
the water, so that no alteration in the free gas can ensue from the combination of the 
chlorine and hydrogen dissolved in the water. 
(8) The volume of uninsolated gas which enters the vessel i from the horizontal tube 
ss during the observations, is very small compared with the total volume of gas exposed 
to the light. In most of the experiments it amounted to about 5-^00 of the total volume 
for every division on the observation- tube. 
(9) The disturbing action of radiant heat is fully eliminated. 
Ha\'ing thus explained the arrangement of our instrument, we now pass to a descrip- 
tion of the experiments instituted for the purpose of testing its efficacy. 
The first action of light on the electrolytic chlorine and hydrogen is accompanied by 
a phenomenon of a very peculiar nature, which we have named Photo-chemical Induc- 
tion, to the fuU consideration of which a special section of this investigation has been 
devoted. On exposing the sensitive gas to a constant source of light, the chemical action 
does not at once commence to its full extent ; to begin with, it is either not perceptible 
or of very slight amount, but gradually increases until a permanent maximum has been 
reached. The following experiment shows this gradual increase of the original action. 
Series of Experiments III. 
Time in minutes 
Corrected observation. 
Action every minute. 
lOO-O 
100-5 
102-1 
102-6 
103-2 
105-3 
119-9 
139-1 
170-2 
200-6 
0 - 5 
1 - 6 
0-5 
0-6 
2-1 
14-6 
29- 2 
31-1 
30 - 4 
In the first minute the action was only 0-5, and after the lapse of eight minutes it had 
reached 31-1, at which it remained constant. The enlargement of the combining power 
of chlorine and hydrogen is also observed, when the amount of light falling on the gas is 
suddenly increased. This peculiar action of the light determines the method which 
must be followed in the measurement of the chemical rays. The observations can never 
be made use of until the successive difierences have become constant. In order to form 
an estimate of the trustworthiness of an experiment from the regularity of the difier- 
ences, and to eliminate as much as possible the slight variations in the working of the 
instrument, it is almost always sufficient to read otf the stand of the liquid every minute, 
and to take the mean of ten or twelve such observations. The folloAving experiment, 
calculated in this manner, gave the mean values as below : — 
3 B 2 
