66 REPORT — 1856. 



The circumstance that the combination of chlorine and hydrogen does 

 not take place immediately on exposure to the light, was observed by Draper 

 in 184'4. This was explained by him on the supposition that the chlorine, 

 by exposure to the light, was transformed into a permanent allotropic modi- 

 fication which differed from ordinary chlorine by possessing greater com- 

 bining power. We have convinced ourselves that this explanation of the 

 phsenomenon is incorrect, and have proved that it is connected with actions 

 of a very peculiar nature which may be classed together under the terra of 

 Chemical Induction. 



A number of experiments made with both diffuse solar and lamp-light, 

 with different mixtures and various masses of sensitive gas for the purpose 

 of determining the inductive action, showed that the times which elapse until 

 the action begins, and until the maximum action is attained, are very different. 

 We therefore next proceeded to examine the various causes which might 

 influence the amount of the induction. First, the relation between the 

 inductive action and the mass of the gas; secondly, the eflFect produced on 

 the inductive action by variation of the amount of light, with a constant 

 volume of gas; thirdly, the effect produced on the inductive action by 

 allowing the gas to remain in the dark ; and fourthly, the action of small 

 quantities of foreign gases upon the induction. 



Experiments carried on with the view of answering the first of these 

 questions, showed that the inductive action, or the transition of the gas from 

 the inactive to the active state, was retarded by increase of the mass of gas. 

 A larger volume of gas had to be insolated for a longer time than a smaller 

 volume before the maximum action ensued. 



The influence of the amount of light on the rate of the inductive action 

 was proved to be very great. The time required for induction diminished 

 with increase of the amount of light, and in a quicker proportion than the 

 increase of light. 



On allowing a sensitive mixture, which had already been insolated, and 

 had attained its maximum action, to stand for some time in the dark, it was 

 found that upon readmission of light the action did not begin again immedi- 

 ately, but a new induction was necessary before the maximum action was 

 attained. Hence the change effected upon the gas by the light is not a per- 

 manent one, for after the light is withdrawn, the gas returns to its original 

 inactive state, and requires as long an insolation before the maximum action 

 is again reached as in the case with the original gas. This fact is of itself 

 sufficient to disprove Draper's statement that this active condition of the gas 

 when once brought about by the action of light is permanent. We have 

 also convinced ourselves by experiment, that the supposition of a non-per- 

 manent allotropic modification of either gas as an explanation of this phae- 

 nomenon is untenable. The gases evolved by the electrolysis of hydrochloric 

 acid were collected separately, and after each gas had separately traversed a 

 tube which could be exposed to direct solar rays, the gases were allowed to 

 mix, and were then passed into the apparatus. On examining the action of 

 lamp-light on the mixture, no difference in the rapidity of the action could 

 be perceived between the sensitive gas, the constituents of which had been 

 separately exposed to direct sunlight, and that which had not been previously 

 insolated. From these experiments it is seen that the explanation of the 

 phsenomenon of photochemical induction is not to be sought iu any 

 allotropic modification of either gas. 



The effect produced by the presence of small traces of foreign gases upon 

 the induction is very remarkable. We have found that the sensibility of 

 the gaseous mixture depends entirely upon the absence of every trace of 



