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duced no afcertainable interval of time betweenthe caufe and effect, which is not furpriling,fince it is highly probable that the motion ofcalorific rays is full as rapid as thofe of light.

Thefe moll; beautiful experiments, with manyothers on the propagation of heat, have beenrepeated with various modifications by Mr.Leflie, f with an apparatus of equal fimplicityand elegance, and with thermometers of apeculiar conftruction and admirable fenfibility.

The important difcoveries contained in thisinterefting refearch make it necefl'ary to extendthe fubjedt of the propagation of radiant heat tothe confideration of the power or aptitude of(comparatively) hot bodies to propel or dif-charge heat in a radiant form from their fur-faces, and of (comparatively) cold bodies toreceive and abforb caloric radiating to their fur-faces. In the former cafe caloric paffes out ofa combined to a radiated Hate; in the latter,the contrary takes place. The beautiful experi-ments which we are about to relate fhew a veryuniform agreement between thefe two contrarypowers, and prove that the power of abforbingradiant heat poffeffed by different bodies or fur-,faces of bodies, is exactly equivalent to theirpower of tranfmitting heat in a radiant form.

The apparatus ufed by Mr. Leflie was thefollowing. A thermometer with a double bulb,conftru&ed in fuch a manner that a rife or fallon the fcale would take place only when theindicative bulb was expofed to a higher or lowertemperature than the other bulb, or than thefurrounding air. This the inventor terms aDifferential Thermometer, and its con-ftrudtion will be explained under that article.The peculiar advantage which it pofleffes, befidesits extreme fenfibility, is, that in ufing it thecommon temperature of the furrounding airmay in general be difregarded, this being alwaysthe zero of the fcale, whatever be the actualvariation of heat in the furrounding atmofphere :and hence a much greater fimplicity is intro-duced in thefe delicate refearches. For thereflection of heat, mirrors were employed, ge-nerally of block-tin, highly polifhed, and ham-mered to fit a wooden gage, the fegment ofa parabolic curve, whereby much of the dif-perfion produced by a Ample concave form wasavoided, fo that when expofed to the direct raysof the fun they collected them into a prettydiflin£t focus of about half an inch in diameter.Fhe fubftance employed to generate the radiantheat was a hollow cubic tin canifter, whofeftation was direftly in front of the mirror of

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its focal point, and when ufed, was filled withboiling water, and fitted with a common ther-mometer paffing through a hole in the cover andimmerfed in the water. The cubical form ofthe canifter allowed of four fides of perfectlyequal dimenfions, each of which, when turnedto the mirror, afforded a heated furface for thetranfmiffion of radiant caloric, and they wereoccafionally coated with various fubftances toafcertain the effect of colour, polifli, and thelike, in retarding or promoting the radiation ofthe heat within.

Some of the principal experiments we finalfrelate.

The bulb of the thermometer being in thefocus of the mirror, if the canifter is filled withboiling water and fet oppofite to die mirror,generally at the diftance of three feet, the liquorof the thermometer begins to rife immediately,and in a few minutes has attained its greateftpoint of elevation, after which it flowly finks asthe water cools, till at laft it returns, in perhapsabout two hours, to die ftandard of the atmof-phere, at which point, whatever be the actualtemperature, the differential thermometer re-mains at the zero of the fcale. This experiment,like that of M. Pi£tet as above related, Ihewsthe fimple reflection of caloric radiating fromthe furface of the heated canifter. With a fix-inch canifter, three feet from the mirror, thetotal rife of the differential thermometer wasabout equivalent to 14.5 0 degrees of Fahrenheit.Mr. Leflie alfo found that in all cafes the fulleffect of the radiant heat on his thermometerwas exactly proportioned to the heat of thecanifter above that of the room. Thus if thewater in the canifter is at 200 ° and the room at60° (making a difference of 140), and the fullrife on the differential thermometer amounts to84 degrees of any fcale, a difference of 100degrees will caufe a rife of 6o° in the famethermometer, for 140 is to 84 as 100 to 60.

The effect: of altering the furface of the heatedbody whence the radiations of heat originate,is fliewn in a very ftriking manner by the fol-lowing experiments. Of the four equal fidesof the cubic tin canifter, one was covered withlamp-black, a fecond with writing paper, a.third with glafs cemented on by pitch, andthe fourth left unaltered. The canifter beingfilled with boiling water, and each fide beingsturned fucceflively. to the mirror, the effects,were furprifingly different. The firft or black-ened fide raifed the thermometer ioo° (of an-arbitrary fcale); the fecond or papered fide,Nature of Heat .