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UR knowledge of the various conflituent principles of natural bodies, goes no farther than their more ftriking effects. The fimilarity of fome of thofe effects, and the diffimilarity of others, point out various particular properties of thofe principles, whence we are enabled to form certain general rules, called laws of nature. Therefore it follows, that with refpect to the effential or fimple state of thofe principles, we can only form conjectures, or offer hypothefes; yet the more circumfcribed nature of fome of them, renders our hypothetical knowledge of their effence more probable, and lefs equivocal, than that of other principles.
Four of the latter fort have, on account of their wonderful effects, and of their very extenfive influence, been fet apart for a more particular examination. Thefe are caloric, light, electricity, and magnetifm.
magnetism. The principal properties of thofe natural agents, the more probable opinions which have. been entertained with refpect to their effence, and the principal advantages which we derive therefrom, will form the contents of the prefent, or third, part of thefe Elements; which, therefore, will be divided into four fections; and each section will be fubdivided into as many chapters as the nature of the fubject may feem to demand, confiftent with perfpicuity and concifenefs.
OF CALORIC; OR, OF THE ELEMENT WHICH PRODUCES HEAT, fire, &c.
GENERAL idea of the element which produces the fenfation of heat, &c. has been given in the preceding volume, wherein the nature of the affinities of the various elements has been concisely illuftrated. In the following pages we muft unavoidably repeat fome of the particulars which have been already mentioned; but the repetition will be short, and the advantage, in point of perfpicuity, will probably prove more than an adequate compenfation for the trouble of twice perufing a few paffages.