220 Royal Society :— 
January 15.—The Lord Wrottesley, President, in the Chair. 
The following communication was read :— 
‘« Photo-chemical Researches.—Part II. Phenomena of Photo- 
chemical Induction.”” By Prof. Bunsen of Heidelberg, and Henry 
Enfield Roscoe, B.A., Ph.D. 
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 
by Draper in 1843. An explanation of this fact was given by the 
supposition that the chlorine underwent on insolation a permanent 
allotropic modification, in which state it possessed more than usually 
active properties. ‘This explanation is, as we shall show, erroneous, 
and the whole phenomenon is caused by the peculiar action to which 
we have given the name of photo-chemical induction. When the 
standard mixture of chlorine and hydrogen is exposed to a constant 
source of light, no action is at first perceptible; after a short time, 
however, the action becomes visible, and gradually increases until a 
constant maximum is reached. Experiments made with different 
amounts of light from different lumimous sources, showed that the 
times which elapsed from the beginning of the exposure until the 
maximum was obtained, varied very much, according to circum- 
stances. In one case the maximum action was reached in fifteen 
minutes, in others after an exposure of three and four minutes. 
The first action was in one case visible after six minutes’ insolation, 
in others after one minute, whilst in some experiments a considerable 
action was observed in the first minute. 
The condition modifying the action of the induction which we 
first examined, was the action of the mass of the insolated gas. From 
various experiments, it was found that the duration of the induction 
