CAL

CAL

( 186 )

So that the heat of the outfide of any veflel tothe touch is no other criterion of the eafe withwhich heat radiates from it, than to make itprobable that the furfaces which feel the hotteftare from their nature thofe which retain heatthe longeft In the air, or any elaftic medium.

The thicknefs of any veflel, more efpecially ifas much as poflible of it is made of light fpongymaterials, diredtly diminifhes the conductingpower, though it often encreafes the radiating.Thus a metallic veflel will cool fafter than beforeif wrapped round with a Angle fold of thinflannel, clofely adhering to the furface, as willbe feen by the following Ample experiment.A thin hollow cylinder of tin-plate, of 2 inchesdiameter and 5 inches in height, quite brightand clean on the outflde, was filled with hotwater, and a thermometer bulb introduced inthe middle of the water through a hole in thecork which fitted to the top. The heat of thewater obferved every five minutes from 195 0was, fucceflively, 195 0 —187.5°—180 0 —173 0 .The fame experiment repeated with every cir-cumftance the fame, except that the cylinderwas clofely wrapped round with a Angle fold offlannel, now gave 195 0 —186 0 —177.5—170°.So that in fifteen minutes the cylinder coveredwith flannel had loft 3 degrees more of heatthan when uncovered. This is explained fromthe principle that the furface is changed from ametallic, to one of loft vegetable matter, whichradiates heat with much more intenfity: but ifwrapped round with feveral folds of flannelloofely adhering, this encreafed radiating powerwould be fully or more than counterbalanced bythe greater difficulty which the enclofed heatwould find in being conducted outwards throughfo great a thicknefs of a loofe fpongy material.

All that has been faid about the power offurfaces to promote or retard the radiation ofcaloric from them when hotter than the fur-rounding air (and equally the abforption andreflexion of radiated caloric) apply exadtly,changing the circumftances, to the radiation ofcold, meaning by this term a temperature lowerthan that of the furrounding atmofphere. Thusiced water contained in a bright clean metallicveflel and expofed to a warm air, retains itseold longer (or, were cold, a pofitive fubflancewe fhould fay the metallic furface propels frigo-riflc rctyj with lefs force) than if the outfide ofthe veflel were painted or even tarnifted. Butthis leads to the curious enquiry relating to

The Radiation and Reflexion of Cold.

Nothing appears at firft fight more directly

contradictory to the common opinion of cold'being only relative, and only a negative termimplying the abftraction of heat, than the fadtswhich ffiew the apparent radiation, abforptionand reflexion of cold; the evidence of whichHands on the fame ground a9 the correfpondingmotions of heat already defcribed, namely, onthe rife or fall of the thermometer. If the rifeof the liquor on the fcale of a thermometerwhofe bulb is placed in the focus of a mirror,be confidered as a proof of the propulfion ofcertain calorific rays from a diftant heated fur-face, and their fabfequent reflexion accordingto the laws of catoptrics; the finking of the famethermometer liquor under fimilar circumftancesof pofition, when the furface which before was-,fenlibly hotter than the atmofphere is nowfenfibly colder, would feem from a parity ofreafoning to indicate the propulfion and reflexionof frigorific rays. Nor can we confider thisqueftion as at all determined, though an in-genious hypothefis has been advanced by MlPrevoft, which goes a confiderable way to recon-cile the apparent contradiction of the dodtrineof the unity of heat and cold; The leadingfafts however are the following.

It is Angular that the reflexion of cold fhouldhave been accidentally, difcovered and decidedlyannounced about the year 1667 by the membersof the Florentine Academy del Cimento, with-out any further profecution of fo curious afadt.

The experiment- is the following. 1 A mafsof ice of about 5001b. was fet fome diftancebefore a concave glafs mirror, and the bulb ofa fpirit thermometer put in the focus to trywhether cold would be reflected. Immediatelythe fpirit of the thermometer began to fink, andfell feveral degrees. To prove that this wasnot merely owing to the contiguity of the ice;the furface of the mirror was covered with acloth to prevent the reflexion, and the ther-mometer again rofe. No further inference isdrawn from this experiment, and the author ofit feemed even to doubt of the reality of thereflexion, and to be difpofed to impute it tofome other unknown caufe.

This experiment was repeated in a muchmore accurate way by M. Piclet. m The appa-ratus which he ufed was the fame as that beforedefcribed, as employed for the reflexion ofheat; that is, two tin mirrors placed diredllyoppofite each other at fome diftance, in thefocus of one of which wjs placed the bulb of avery fenfible thermometer, and in the other tire" Effai fur 1c Feu,