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 \vas then darkened for one minute, and 

 then again iusolated, 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 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 ; 

 but in this communication we restrict ourselves to the consideration 

 of these phenomena in so far as they influence the action of photo- 

 chemical induction, intending on a future occasion to enter more 

 fully into the new field of research indicated. 



The contact action of foreign gases is still more strongly seen in 



