Philosophical Transactions of the Royal Society of London

    Affinity, or the force which causes the particles of different bodies to unite to form a chemical compound, is, both as regards its nature 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 circumstances 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 ff their chemical forces, whilst at another they are prevented from so doing by other forces acting in an opposite direction. These opposite attractions, which must be overcome in order that the chemical combination should take place, may he 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 magnitude 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.

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