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January 15, 1857. 



The LORD WROTTESLEY, President, in the Chair. 

 The following communications were read : 



I. " Photo-chemical Researches. Part II. Phenomena of 

 Photo-chemical Induction." By Prof. BUNSEN of Heidel- 

 berg, and HENRY ENFTELD ROSCOE, B.A., Ph.D. Com- 

 municated by Prof, STOKES, Sec. R.S. Received November 

 27, 1856. 



(Abstract.) 



Chemical affinity, or the force which regulates the chemical com- 

 bination of two bodies, is like all other forces, a certain definite 

 quantity. Hence it is erroneous to say, that under different circum- 

 stances the same body can possess different affinities ; more correctly 

 we should say, that in the one case the bodies are able to follow the 

 chemical attraction of their molecules, whilst in another case opposing 

 forces render this combination impossible. These opposing forces 

 may be considered as resistances similar to those exerted in the 

 passage of electricity through conductors, in the distribution of 

 magnetism in steel, and in the conduction of heat. We overcome 

 these resistances when by agitation we increase the formation of a 

 precipitate, or by insolation effect a decomposition. 



We call the act by which these resistances to combination are 

 lessened, and the formation of a chemical compound promoted, 

 "chemical induction;" and we specify this as photo-chemical, thermo- 

 chemical, electro-chemical, or idio-chemical, according as light, heat, 

 electricity, or pure chemical action is the force which promotes the 

 combination. 



The phenomena of photo-chemical induction are particularly 

 interesting, as affording starting-points from which we may gain a 

 knowledge of this mode of action of affinity. 



That on exposing a mixture of chlorine and hydrogen to the light 

 the action does not commence to its full extent at once, was observed 



