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UR knowledge of the various constituent
principles of natural bodies, goes no farther than their more striking effects. The finilarity of some of those effects, and the diffimilarity of others, point out various particular properties of those principles, whence we are enabled to form certain general rules, called laws of nature. Therefore it follows, that with respect to the essential or simple state of those principles, we can only form conjectures, or offer hypotheses; yet the more circumfcribed nature of fome of them, renders our hypothetical knowledge of their effence more probable, and less equivocal, than that of other principles.
Four of the latter fort have, on account of their wonderful effects, and of their very extensive infuence, been fet apart for a more particular examiration. These are calcric, light, ele&tricity, and
magnetism. The principal properties of those natural agents, the more probable opinions which have been entertained with respect to their essence, and the principal advantages which we derive therefrom, will form the contents of the present, or third, part of these Elements; which, therefore, will be divided into four sections, and each section will be subdivided into as many chapters as the nature of the subject may seem to demand, consistent with perspicuity and conciseness.
OF CALORIC; OR, OF THE ELEMENT WHICH PRO
DUCES HEAT, FIRE, &c.
GENERAL idea of the element which pro
duces the sensation of heat, &c. has been given in the preceding volume, wherein the nature of the affinities of the various elements has been concisely illustrated. In the following pages we must unavoidably repeat some of the particulars which have been already mentioned; but the repetition will be short, and the advantage, in point of perspicuity, will probably prove more than an adequate compensation for the trouble of twice perufing a few passages.