PEOFESSOE BWSEN AFTD DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 387 
In each of the four experiments with varying amount of light, the first column, gives 
the length of exposure expressed in minutes, the second and third columns, h and c, the 
amount of action expressed by the volume of hydrochloric acid formed each minute, and 
the fourth column, fZ, the mean of h and c. 
The experiment in which the amount of light was equal to I'O, proves that the action 
began to show itself after the gas had been exposed for five minutes ; with the amount 
of fight 1'78 the action began in the first minute; with the amount of fight 2 ‘45 a 
considerable action was attained dining the first minute, and when the intensity of 
the fight was 4T7, more than half the amount of the maximum action was attained 
in the first min ute. When the intensity of the fight was equal to I’OO, the maximum 
action was reached after nine minutes’ exposure, with intensity 1-78 after five minutes, 
with intensity 2 ‘45 after from three to four minutes, and with intensity 4T7 after three 
minutes. The difierences between the observations show that the photo-chemical in- 
duction does not vary proportionally with the times of exposure. With intensity 1, the 
action increased in the first minute 2T divisions, in the following minute 12 '5, in the 
thfixl 14-6, and in the fourth 2'2 ; whilst with intensity 1-78 it increased 2T in the first 
minute, 8'7 in the second, 22-3 in the third, 20-6 in the fourth, and 2‘5 in the fifth, and 
in a similar way for the other intensities. 
These experiments show, — 
(1) That the time of exposure necessary to effect the first action of the photo-che- 
mical induction decreases with the increase of the fight, and in a greater ratio than the 
increase of fight. 
(2) That the time which elapses until the maximum action is attained also decreases 
with increase of fight, but in a much less ratio than the increase of fight. 
(3) That the increase of the induction proceeds at first in an expanding series, attains 
a maximum, and again converges until the constant action is attained. 
The curves, Plate XIX. fig. II., of which the abscissse represent the duration of the in- 
solation, and the ordinates the corresponding action expressed in volumes of hydrochloric 
acid formed, are obtained from the mean d of the two series of observations. They 
show the law which regulates the decrease of the resistance to combination of the chlo- 
rine and hydrogen mixture with increasing amounts of fight. 
The results of these experiments suggested the question. Is this condition of increased 
combining power, into which the mixture of chlorine and hydrogen passes by insolation, 
permanent, or is it merely confined to the space of time during which the gas is exposed^ 
to the fight \ 
The following was the method employed for determining this important point. We filled 
the apparatus (Plate XVII. fig. 2) with miinsolated sensitive gas, and observed the in- 
crease of the induction during every 30", until the maximum action was attained. The 
insolation-vessel was then darkened for one minute, and after lapse of this time the action 
was again observed until the maximum was attained. These observations were repeated 
several times, each period of darkening being longer than the preceding. 
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