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by Draper in 1843. An explanation of this fact was given by the 

 supposition that the chlorine underwent on insolation a permanent 

 allotropic modification, in which state it possessed more than usually 

 active properties. This explanation is, as we shall show, erroneous, 

 and the whole phenomenon is caused by the peculiar action to which 

 we have given the name of photo-chemical induction. "When the 

 standard mixture of chlorine and hydrogen is exposed to a constant 

 source of light, no action is at first perceptible ; after a short time, 

 however, the action becomes visible, and gradually increases until a 

 constant maximum is reached. Experiments made with different 

 amounts of light from different luminous sources, showed that the 

 times which elapsed from the beginning of the exposure until the 

 maximum was obtained, varied very much, according to circum- 

 stances. In one case the maximum action was reached in fifteen 

 minutes, in others after an exposure of three and four minutes. 

 The first action was in one case visible after six minutes' insolation, 

 in others after one minute, whilst in some experiments a considerable 

 action was observed in the first minute. 



The condition modifying the action of the induction which we 

 first examined, was the action of the mass of the insolated gas. From 

 various experiments, it was found that the duration of the induction 

 increased with the volume of exposed gas (by constant amounts of 

 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 



