Royal Institution. 483 



many facts concerning the chemical action of light, this branch of 

 science has onlj' as yet arrived at that first or qualitative stage of 

 development, through which every science must pass. The laws 

 which reaiulate these phenomena are unknown to us, and we possess 

 no means of accurately measuring the amount of the decomposition 

 effected by the light. 



The speaker proceeded to describe the results of a series of 

 experiments carried on by him in conjunction with Professor Bun- 

 sen, which had for their object, — 



1. To determine the lawswhich regulate the chemical action of light; 



2. To obtain a measure for the chemically active rays. 



When aqueous solutions of chlorine, bromine, or iodine are ex- 

 posed (under certain conditions) to the direct solar rays they are 

 decomposed, the corresponding hydracid being formed, and the oxy- 

 gen of the water liberated. The difference between the amounts of 

 free chlorine, bromine, or iodine, contained in the liquid before and 

 after exposure to light, gives the quantity of the substance decom- 

 posed during the isolation. Now it was found that this quantity of 

 chlorine, bromine, or iodine which disappeared, was not proportional 

 to the time of exposure to the light ; in twice the time, for instance, 

 less than twice as much substance was decomposed. 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 phsenomena show this 

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

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

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

 formed, lience 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 si' nld 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 cond)ination is immediately absorbed, and thus with- 

 drawn Irom the sphere of action, and the diminution of the volume 

 of the mixed gases arising from this absorption gives an exact mea- 

 * Poggendorff's Annulen, xcvi. 'SJ-i; nnd Quarterly Journal of Chemical 

 Society, Oct. IHSo. 



