PEOFESSOE BIJNSEN AjS^D DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 391 
As we have shown that expansion from so small an increase of temperature exerts n(; 
perceptible influence on the combining power of the chlorine mixture, the force which 
has diminished this combining poAver from 43‘47 to 16'4 must be a chemical force pro- 
ceeding from the particles of the hydrogen. 
In these experiments we have to do with the purest form of the phenomena which are 
classed under the name of catalysis, freed from all foreign disturbing causes. We thus 
arrive on the special field of catalytic phenomena to which the chemical action of light 
is most closely related. In order fully to understand these relations, we must first form 
a clear idea of the nature of these contact-actions. 
All chemists agree that the phenomena of affinity depend upon the specific attractions 
which exist between the particles of bodies of different natures. These attractions must 
necessarily exist when the particles are prevented from folloAving them to form a chemical 
compound. Let us suppose the particles a and h so brought together that a chemical 
attraction is exerted between them ; and let us suppose a third body, c, brought into the 
sphere of attraction of the other two ; this third body will then also exert an attraction 
upon a and b. The attraction between a and b Avill not remain the same as it originally 
was, but it will be the resultant of all the forces originating in a, b and c. It is thus 
easily seen, that the attractions, which tend either to effect or support a chemical combi- 
nation between Lvo bodies, must be altered in the sphere of action of a third body ; and 
that the presence of a third body may therefore, according to ch’cumstances, effect or 
preA’ent the formation of a chemical compound. From this point of vieAV the explana- 
tion of contact phenomena offers no difficulties. The point which has in general been 
thought to be the most inexplicable, is the fact that the smallest given quantity of one 
substance is able to combine or decompose the largest given quantity of another sub- 
stance, mthout thereby losing any of its decomposing or combining power ; as if an 
amount of effect could be produced without the expenditure of an equivalent amount of 
force. We need, however, only consider a little more closely the conditions under which 
the phenomena of contact occur, in order to assure ourselves that these phenomena do 
not \iolate this first law of mechanics, but that they are merely common and simple 
effects of chemical affinity. If Ave suppose a number of chemically combined particles 
placed in the neighbourhood of a contact-substance, the attraction of the foreign contact- 
body is added to that AA’hich holds the chemically combined substances together, and the 
resultant of all these forces may be such that a decomposition of the chemically com- 
bined particles can ensue. AATien the decomposition has been effected, and the contact- 
substance is still surrounded by the separated particles, all further action must cease. If, 
hoAveA^er, these separated particles are removed, and undecomposed molecules arranged 
in their stead, the phenomenon just described aaIII be repeated ad infinitum as often as 
new particles are brought in place of the old ones. This removal and replacement of 
the molecules undergoing catalysis, and the expenditure of force connected thereAvith, 
furnishes, hoAveA^er, the equivalent of force for that exerted by the contact-substance. 
For as the substance produced by the catalytic action either sinks to the ground on 
account of its greater specific gravity, or is vaporized by assumption of heat, escapes by 
