JSaeal fttijstitution of ffireat Utitain 



WEEKLY EVENING MEETING, 



Friday, April 4, 



Sir Henry Holland, Bart. M.D. F.II.S. Vice-President, 

 in the Chair. 



Henry E. Roscoe, Esq. B,A. Ph.D. 

 On the Measurement of the Chemical Action of Light. 



No attempt has been made, up to the present time, accurately to 

 measure the changes brought about in chemical substances by the 

 action of the solar rays. 



The peculiar action of light on chemical bodies was first ob- 

 served by Scheele on chloride of silver. Since that time the 

 subject of the chemical action of light has attracted a large amount 

 of attention, as the present perfection of the arts of the daguerreo- 

 typist and photographer fully testify. Although we possess so 

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

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

 developement, through which every science must pass. The la\^s 

 which regulate 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 tjae 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 laws which 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 



exposed (under certain conditions) to the direct solar rays they 

 are decomposed, the corresponding hydracid being formed, and 

 the oxygen 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 decomposed during the insolation. 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 dc' 

 Vol. II.--(No 23.) q 



