CHEMICAL

The attractive force by which di imilar substances combine with each other to produce chemical compounds. All natural and artificial substances are either simple or compound. The metals, for instance, are simple substances, — no one of them having been as yet decomposed: water is a compound; it may be resolved into oxygen and hydrogen gases, which are therefore called its component parts. To enable substances to exert their mutual affinities, or to act chemically upon each other, the opposing powers of matter must be overcome, and they must be placed under circumstances favourable to the exertion of their mutual chemical attractions. Two solid bodies seldom combine, in consequence of their imperfect contact, and the imirobility of their particles; hence the old axiom, corpora non agunt, nisifltiida. But to this there are exceptions: ice and salt, for instance, run down into liquid brine; oxalic add and dry lime unite; and when suljihur and chlorate of potash are rubbed together, they act violently on each other. Even when one or both substances are fluid, heat is often requisite to diminish cohesion, and promote affinity: thus, mercury and iron combine with melted sulphur; and oxygen and hydr»< C 2 [s. 32]