iNTi;oircTiON 27 



As this t'< >ivc must be regarded as exclusively an Attractive force, 

 like gravity, many writers (for instance, Berginanii at the end of the 

 last, and Berthollet at the beginning of this, century) supposed affinity 

 to be essentially similar to the universal force of gravity, from which 

 it only differs in that the latter acts at observable distances whilst 

 affinity only evinces itself at the smallest possible distances. But 

 chemical affinity cannot be entirely identified with the universal 

 at traction of gravity, which acts at observable distances and which 

 is dependent only on mass and distance, and not on the quality of the 

 material on which it acts, whilst it is by the quality of matter that 

 affinity is most forcibly influenced. Neither can it be entirely identi- 

 fied with cohesion, which gives to homogeneous solid substances their 

 crystalline form, elasticity, hardness, ductility, and other properties, 

 and to liquids their surface, drop formation, capillarity, and other 

 properties, because affinity acts between the component parts of a 

 substance and cohesion on a substance in its homogeneity, although 

 both act at imperceptible distances (by contact) and have much in 

 common. Chemical force, which makes one substance penetrate into 

 another, cannot be entirely identified with even those attracting 

 forces which make different substances adhere to each other, or hold 

 together (as when two plane-polished surfaces of solid substances are 

 brought into close contact), or which cause liquids to soak into solids, 

 or adhere to their surfaces, or gases and vapours to condense on the sur- 

 faces of solids. These forces must not be confounded with chemical 

 forces, which cause one substance to .penetrate into the substance of 

 another and to form a new substance, which is not the case with 

 cohesion. But it is evident that the forces which determine cohesion 

 form a connecting-link between mechanical and chemical forces, be- 

 cause they only act by intimate contact and between different kinds of 

 matter. For a long time, and especially during the first half of this 

 century, chemical attraction and chemical forces were identified with 

 electrical forces. There is certainly an intimate relation between them, 

 for electricity is evolved in chemical reactions, and it, in its turn, has 

 a powerful influence on chemical processes for instance, compounds 

 are decomposed by the action of an electrical current. But the exactly 

 similar relation which exists between chemical phenomena and the 

 phenomena of heat (heat being developed by chemical phenomena, and 

 heat being able to decompose compounds) only proves the unity of the 

 forces of nature, the capability of one force to produce and to be trans- 

 stances combining have something in common a medium. As is generally the case, 

 another idea evolved itself in antiquity, and has lived until now, side by side with the 

 first, to which it is exactly contradictory ; this considers union as dependent on con- 

 trast, on polar difference, on an effort to fill up a want. 



