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438 inflammable substances. Seft. 228.. being mixed with heterogeneous bodies: that fmali the acid receives the heat, not the phlogifion. Even the various coloured rays of light contain unequal fhares of phlogifion, fince the violet rays part more eafily with their phlogifion, than the other rays, to revive metals. When light is not obftruc- ted in its paffage, no heat is. perceived ; but if Sopped in it* courfe, the oppofing body receives heat, aud fometimes phlogifion. Light feems therefore to be the matter of heat loaded with a fuperabundant quantity of phlogi/lon. That which comes out from a furnace, produces heat on the fur- rounding bodies, which afcends up with the rarified air; proceeds forwards in ftraight lines: and may be refletled front polilhed furfaces, with this peculiarity, that a concave glafs- mirror retains the heat, whilfl it reflefls the light; for, although, its focus is bright, yet it is not warm. A plane of glafs inter, pofed to this light, retains the heat, and becomes hot, but lets the light pafs through. Fire is the more or lei's heated, and more or lefs luminous flate ■ of bodies, by which they are refolved into their condiment parts, and entirely deftroyed. It requires that they be previoufly heated in contaft with air ; for, to every combujlible body, a certain quantity of heat muft be communicated, in order to fet it in the fiery commotion. Combujlion is the aftion of the heat penetrating the pores of bodies, and deftroying their cohefion; in this cafe the body parts with its phlogifion, provided there be a fubftance prefent, which has a firong attradlion to the inflammable principle. If the heating be performed is open air, the em pyreal air (which makes one-fourth part of the bulk of the atmofphere), on account of its ftronger attra<flion, unites with the inflammable principle, which is thus fet at liberty ; from this union the heat is compounded; and fcarcely is this heat generated, when the combuftible body is ftill more expanded by it than in the beginning, and its phlogifion more laid open. The more the heat is increafed, the more minute are the particles into which the combujlible body is difiolved. The empyreal air meets more furfaces, confequcntly cornea in contadt with mere phlogifion ; and, according to its na ture,
