V

PREFACE.

The chemical attraction, or affinity, obtains betweenbodies as being composed of parts, and as being of a diffe-rent species of matter from one another. It never takes* place while the two bodies are at any sensible distance j

and when they are brought into the closest contact, thereis frequently necessary some other power, as fire, to excitetheir action upon one another. In proportion as thisaction happens, they are no longer two bodies, but one zthe affinity consisting in the intimate coalition of the partsof one body with those of the other. The properties ofthis new compound are not in any kind of ratio of thoseof the compounding bodies, nor discoverable by any ma-thematical investigation : two bodies, each by itself veryeasily fusible in the fire, as lead and sulphur, shall form acompound very difficult of fusion ; and two which cannotseparately be made to melt at all, as pure clay and chalk,shall melt with ease when joined together.

As the chemical union, and the properties thence re-sulting, are exempt from all known mechanism, so neithercan the bodies be separated again by mechanic force. Buta third body may have a stronger affinity to either of thecomponent matters than they have to one another, iuwhich case, on presenting to the compound this third body,the former union is broken, and one of the first bodiescoalesces with the third, while the other is detached andseparated.

Thus, when quicklime is dissolved in water, if we addto the transparent fluid a little vitriolic acid, the acid par-ticles unite with the dissolved particles of the lime into a* a L new