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-about 98°; the third, or glazed, gave 90°; butthe poliflied tin fide gave only 12 0 . From thiscurious experiment it appears that a polifhedmetallic furface has no more than about aneighth part of the power of propelling heatpoflefled by a foft unpolifhed furface.

To Ihew that this difference of the tranf-mitting or propelling power was correlative withthat of the receptive power, as abovementionedto be the cafe, the bulb of the thermometerwas now coated with tinfoil preffed fmoothlydown, fo as to imitate very exactly the polifhedfurface of the canifter. The blackened furfaceof the canifter being now prefented to the mir-ror (which produced an effect of 100 degreeson the plain bulb of the thermometer) nowraifed it only 20* ; and when the plain furfaceof the canifter was prefented (which before hadgiven 12 0 ) it now gave only about 2-5°.

The inference from thefe experiments is,firft, that the propelling and abforbing powersof furfaces relatively to radiant caloric are alwaysin the fame proportion, and that this power ismany times lefs in bright metallic furfaces, thaneven in glafs, and ftill more in foft unpolifhedfurfaces i'uch as black paint or paper.

The power of abforbing, and of propelling ortranfmitting heat, being coexiftent and correla-tive in the fame fubftance, they muft both bein direct oppofition to that of reflecting heat,the furfaces which the moft oppofe the receptionand propulfion of heat being thofe which reflectit the moft powerfully. The experiments juftrelated ftiew that the poliflied metallic furfaceof the canifter with difficulty tranfmits caloric,and that the metallic coating of the thermometerbulb prevents its abforption of heat; the fol-lowing experiments fhew how much fuperiorpower of refleSHttg heat is poflefled by a metallicover a glazed or any other furface.

In the place of the tin reflector (of the appa-ratus before deferibed) fubftitute a concave glafsmirror, and put the. naked thermometer-bulb inIts focus as before; then fill the canifter withboiling water and prefent the blackened fide.The effect will now be very fmall, but deter-minate. Rub oft' the fftvering from the back ofthe mirror, and the eftedt remains the fame.Roughen the back by grinding with emery, thefame effect is produced. Cover the anteriorfurface of the .mirror with Indian ink, and alleffect on the thermometer ceafes. Cover theface of the mirror with tinfoil, and the rife ofthe thermometer will now be ten times as muchas with the fmooth furface of glafs. The famedifference between the reflecting power of glafs

and that of metal had been obferved by M.Pictet, who ingenioufly fuppofes that the radiaptheat firft penetrates the anterior or glafs furfaceof the mirror, and proceeds (much obltructed)to the metallic coating on the back, from whichit is refledted and pafl'es back out of the anteriorfurface into the atmofphere. Hence the greatlofs of heat is accounted for by its double paf-fage through the entire thicknefs of glafs. Butthe circumftance that the reflefting power con-tinues juft the fame with or without the filver-ing of the mirror at the back deftroys this hypo-thefts, and hence the explanation will be, thatglafs being a better abforber of radiant caloricthan polifhed metal, takes in the greater part,and reflects only a fmall portion from the fur-face, againft which the radiant heat firft ftrikes.Hence too by covering the glafs with a foftcoating of black ink, the abforbing power isftill more encreafed, and confequently the re-flecting power proportionably diminifhed.

No delicacy of experiment is required to fhewin a general way the vaft fuperiority of metalover glafs in reflecting heat. Thus if the com-moneft tin reflector, at all approaching to aconcave or parabolic form, be held feveral feetfrom a large fire, and the finger put in thefocus, a very confiderable rvarmth will be per-ceived; whereas the beft glafe mirror of thefame dimenfions and at the fame diftance, willhardly collect in its focus heat enough to befelt.

The following fcale of effect produced by theabove apparatus of Mr. Leflie; will ferve to givea clear idea of the fubjeCt. Oppofe the brightfide of the tin canifter filled with hot water toa glafs mirror, and coat the thermometer-bulbwith tinfoil. Here the canifter, the mirror, andthe bulb are all in the moft unfavourable cir-cumftances for the tranfmiftion, reflection, andabforption of radiant caloric. The rife of thethermometer will be very fmall, call it equiva-lent to 1 degree. Then cover the face of theglafs mirror with tin-foil, which will have thefame effedt as fubftituting a metallic mirror, andthis reflexion will be fo much encreafed thatthe thermometer will rife to io°. Then take oft’the tinfoil from the bulb, whereby its abforbingpower will be encreafed, and it will rife to 50°.Laftly, prefent the blackened fide of the canif-ter, which tranfmits caloric with the moft eafe,and the full effect will now be 400°, as everycircumftance is now the moft favourable to thetranfmiflion and reflection of heat.

For experiments where great nicety is notrequired, as polifhed metallic fpecula are