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and confequently propels caloric with muchlefs force than when both outer furfaces werepainted.

Place each half-blackened tin fcreen on fepa-rate frames; join them together, and the effecton the thermometer will be 23 0 ; put them twoinches afunder, and the effect will not be per-ceptible. The fcreen die furtheft from thecanifter in this cafe only receives a doubly deri-vative caloric (being propelled from the firftfcreen, which is itfelf a fecondary fource) whichis fo much enfeebled in its paffage as to beimperceptible by the thermometer.

From a number of experiments it is obviousthat heat radiates from the furface of the pro-pellent body, like light, not only in perpen-dicular but in divergent lines. It becomes aqueftion whether the radiating or propulfiveenergy is at all modified by the direction whichit takes from the propellent furface. Mr. Lefliefound that the effect of a heated cubic canifterwas the greateft, when the radiating fide wasexactly parallel to the mirror, and decreafedgradually when an oblique direction was givenby turning it on its center, and thus prefervingthe fame diftance. The law of this decreafeappears to be proportional to the degree ofobliquity, whence it may be laid down as ageneral rule that the a£tion of a heated furfaceis proportional to the fine of its inclination, andtherefore is greateft in a direftion perpendicularto the furface.

The times taken up by the refpe&ive coolingof heated bodies expofed to a cold atmofphereor any other elaftic medium, equally illuftrate allthe preceding obfervations concerning the effectof furfaces in promoting or retarding the radi-ation of heat. Thus when a hollow tin ball isfilled with hot water and fet to cool in a tran-quil atmofphere, if it takes 156 minutes to cooldown to a given point when the furface is brightand clean, the fame, painted with lamp-black,will cool to the given point in 81 minutes. hThis indeed does not accord with the propor-tional propulfive energy of thefe two oppofitefurfaces in the experiments of the reflexion ofradiant caloric already mentioned; for in thefeit was found that the force of radiating caloricfrom thefe two furfaces was as 96 to 12, oreight times as ftrong in the blackened as in thebright metallic furface. Whereas the a£tualrate of cooling is only four to one, or fourtimes as great in the painted as in the metallicball. Some other caufe than radiation mufftherefore a£t in this cafe, and mull act nearly

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equally upon both furfaces, fince it tends todiminiih the relative proportions between thecooling power of thefe two fubftances. Thiswill be mentioned in the next feci ion, whentreating of the conducting power. The actuallaw of cooling that regulates all bodies immerfedin a fluid medium, will be prefently noticed.

Radiation of caloric has been obferved totake place only in elaftic mediums, or perhapsin a vacuum, but certainly not in water. Toprove this, if the apparatus of the heated canif-ter and mirror be immerfed in water, no rifewhatever takes place in tire differential ther-mometer when placed in the focal point. ‘ Like-wife, a clean bright tin ball filled with hotwater and immerfed in cold water, cools downto a given point as foon when the furface isbright as when painted.

Several important practical inferences maybe drawn from the properties of radiated caloric,and the power of different furfaces to propel,abforb, and reflect radiated heat. Many of thefeare in direct oppofition to commonly receivedopinions. Thus in cafes where it is requiredto preferve the heat of any fubftance confinedin a metallic veflel, the external furface fliouldbe kept clean, bright, ffnooth and polifhed; *on the other hand, if the object be to cool themquickly, the furface fliould be painted or tar-niflied, or covered with a thin layer of fonaefoft unmetallic coating. For example, metallicpipes intended to convey fleam uncondenfed toa diftance fliould be kept bright, or may advan-tageoufly be gilt, as the thinneft pofiible coat ofgilding will anfwer the purpofe of diminifliing asmuch as pofiible the quantity of radiated caloric.On the contrary, metallic pipes or flues con-veyed into a room with the view of warming it,fhould be rough, unpolifhed, tarnifhed, or better,painted.

Some limit however is put to the calculationof the gain or lofs of caloric by radiation, bythe circumftance that conducted caloric (whichwill prefently be defcribed) in very many cafeshas precifely an oppofite range of action. Thusmetallic bodies conduct heat with much morefacility than glafs or wood of equal thicknefs $and hence when a perfon touches a metallicveflel containing a hot liquid, the heat is forapidly conduBed to the finger as immediately tobecome intolerable; and yet, as has been fullyfhewn, the fame veflel keeps its contained liquidmuch hotter in the air in a given time than aglafs or wooden veflel of the fame dimenfions,which the hand could touch without uneafinefs.

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