Prof. Bunsen and Dr. Roscoe on Photo-che:nical Induction. 221 
increased with the volume of exposed gas (by constant amounts ot 
light), and curves have been drawn, representing the increase of the 
induction for the various volumes of gas employed. 
We next examined the dependence of the duration of induction 
upon the amount of light to which a constant volume of gas was 
exposed, and experiment showed— 
1. That the time necessary to effect the first action of the photo- 
chemical induction decreases with increase of light, and in a greater 
ratio than the increase of light. 
2. That the time which elapses until the maximum is attained 
also decreases with increase of light, but in a much less ratio. 
3. That the increase of the induction proceeds at first in an ex- 
panding series, attains a maximum, and then converges when the 
true maximum action is attained. The law regulating the increase 
of the induction by increase of light, we have rendered visible 
by curves. 
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 
confined to the time during which the gas is exposed to the light? 
In order to determine this question, the sensitive gas, which had 
stood for some time in the dark, was exposed to a constant source of 
light, and the time noted which elapsed before the maximum action 
was reached; the apparatus was then darkened for one minute, and 
then again insolated, and the time watched until the maximum action 
was again observed. These observations were repeated several times, 
each period of darkening being longer than the preceding. Thus 
conducted, the experiment led to the important conclusion, that the 
resistance to combination overcome by the influence of the light is 
soon restored when the gas is allowed to stand in the dark. Curves 
expressing the effect produced on induction by darkening, and by 
exposure to light, have been drawn. 
We have explained the fact, that the mixture of chlorine and 
hydrogen does not combine in the dark, by the supposition of the 
existence of a resistance to combination which is overcome when the 
gaseous mixture is exposed to light. This resistance to combination 
can be increased by various circumstances. The presence of a very 
small quantity of foreign gas in the standard mixture of chlorine and 
hydrogen is sufficient to cause the resistance to be increased to a 
very great extent. An excess of ;,3,5 of hydrogen over that con- 
tained in the normal gas, reduced the action from 100 to 38. 
In these experiments we have to do with the purest form of the 
so-called catalytic actions, to which the photo-chemical phenomena 
are closely related. 'The quantitative estimation of the relations 
which exist in the phenomena of contact, between the mass of the 
substance the time and other modifying conditions, has not as 
yet been possible, owing to the absence of any case in which these 
relations are exhibited in their simplest form. Our method of 
photo-chemical measurement points out a direction which promises 
to afford interesting results concerning these quantitative relations ; 
