224 Mr, Roscoe, on the Measurement of Light. [April 4, 



composed. The relation between the amount of light and the 

 amount of decomposition was found in this case not to be a simple 

 one. 



This anomalous action may be explained even from a theoretical 

 point of view. Chemical affinity is the resultant of all the forces 

 which come into play during the reaction ; hence it is not only the 

 interchanging atoms which influence the result, but also those atoms 

 which, without taking part in the decomposition, surround those 

 actively engaged. The so-called catalytic phenomena show this 

 action in a striking manner. To apply this general principle to the 

 special case before us ; we have to begin with pure chlorine water ; 

 after the first action of the light, however, hydrochloric acid is 

 formed, hence the composition of the solution is altered, and a 

 different result must be expected. This theoretical conclusion was 

 verified by experiment. Chlorine water, to which 10 per cent, of 

 hydrochloric acid was added, did not suffer any decomposition by 

 an exposure of six hours to the direct sunlight ; during which time 

 the same chlorine water, without previous addition of hydrochloric 

 acid, lost nearly all the free chlorine which it contained.* 



In order then to obtain a true measure of the action of light on 

 any chemical substance, it is necessary that the body formed by the 

 decomposition should be removed from the sphere of action. This 

 cannot be done with chlorine water ; a new sensitive substance was 

 therefore employed. 



Equal volumes of chlorine and hydrogen gases when exposed to 

 the direct sun light unite with explosion ; in diffuse light, the action 

 proceeds gradually. In presence of water the hydrochloric acid 

 formed by the combination is immediately absorbed, and thus 

 withdrawn from the sphere of action, and the diminution of the 

 volume of the mixed gases arising from this absorption gives an 

 exact measure of the amount of action effected by the light. The 

 diminution in volume of the gas measured by the rise of water in a 

 graduated tube was found to be regular, proving that when the 

 light is constant the amount of action is directly proportional to the 

 time of exposure. 



The relation between the amount of action and the amount of 

 light was experimentally determined, by allowing known quantities 

 of diffuse light to fall upon the sensitive gas. Experiments thus 

 conducted showed that the amount of action is directly proportional 

 to the amount or intensity of the light. These simple relations 

 were observed by Dr. Draper, of New York, in 1843 ; but his 

 method of experimenting differed essentially from that employed in 

 these researches, and was not susceptible of any very great degree of 

 accuracy. The relation between the amount of action and the mass 

 of the sensitive gas has not as yet been fully determined ; experi- 



♦ See Poggendorff'a Annalen, xcvi., 373 ; and Quarterly Journal of Chemical 

 Society, Oct. 1855. 



