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XVIII. Photo-chemical Researches. — Part II. Phenomena of Photo-chemical Induction. 
By Professor Bunsen, Professor of Chemistry at the University of Ileidelhery^ amd 
Henky Enfield Roscoe, B.A.., Ph.I).., Professor of Chemistry at Owens College, 
Manchester. Commumcated by Professor Stokes, Sec. R.S. 
Eeceived iN’oveinber 27, 1856, — Eead Jaunary 15, 1857. 
Affinity, or the force which causes the particles of different bodies to unite to form a 
chemical compound, is, both as regards its natm’e and quantity, a definite magnitude, 
which, like all other forces and like matter itself, can neither be created nor destroyed. 
It is therefore a badly chosen form of speech to say, that under certain ch'cumstances a 
body attains an affinity which under other circumstances it loses. This expression can only 
be understood to mean, that at one time the bodies are enabled to follow the attraction 
of then* chemical forces, whilst at another they are prevented from so doing by other 
forces acting in an opposite dhection. These opposite attractions, which must be over- 
come in order that the chemical combination should take place, may be presented to the 
mind under the image of a resistance similar to that occurring in friction, in the passage 
of electricity through conductors, in the distribution of magnetism in steel, or in the 
conduction of heat. We overcome this resistance when we quicken the formation of a 
precipitate by agitation, or when, by increase of temperature, by catalytic action, or by 
insolation, we cause a chemical action to occur. 
Such a resistance to combination presupposes a certain combining power, the magni- 
tude of which is measured by the mass of the body chemically combined in the unit of 
time under the influence of the unit of force. 
The act by which the resistance to combination is diminished, and the combining 
power thus brought into greater activity, we call Chemical Induction ; and we specify 
this as photo-chemical, thermo-chemical, electro-chemical, and idio-chemical, according 
as light, heat, electricity, or pure chemical action is the active agent concerned in over- 
coming the resistance. 
The mode of action of affinity, when separated from the influence of these resistances 
to combination, that is, the law which regulates this force when all disturbing causes 
are removed, is entirely unknown. Although the determination of this law, solving the 
the most important problem of the science, appears in the present state of our know- 
ledge stiU to lie in the far distance, it is certainly not premature to search for facts which 
may serve as starting-points in this field of investigation. The interesting relations in 
which photo-chemical induction stands to this question have, therefore, attracted our 
special attention. 
