74 ON HEAT. 



But the fafts This reafoning appears to me to be juft, and if I am not 



are contrary. j • j i 



deceived, (lie conclufions which may be drawn from the facts 



in queftion, well confirmed by experiment, ought to be conli- 



dered as demon ftrative. I have taken every poffible care to 



eftabiifh thefe fads; and the refullsof all my experiments have 



conilantly (hewn that more or lefs perfect polith, or the greater 



or lefs brightnefs of the furface of a metallic veflfel, does not 



fenfibly influence the time of its cooling. 



A policed and I took two equal veflels of brafj arid polillied the external 



Jtiot'btrsc^ed furfacc of onc oi them as hi K h *y as P^^ble ; and I deflroyed 

 in the fame time the polifli of the other by rubbing it 'in all directions with coarfe 

 emery. When thefe two veflels were filled with hot water, I 

 did run jinci that the unpoliflied yefTfci employed more or lefs 

 time in cooling than that which waa polillied. 

 Ciution. I was careful to wafh the furface of the unpoliflied velTel 



effectually with water before the experiment; as I knew that 

 if I did not take the precaution of removing all the dirt which 

 might be lodged in theafperites of (he furface, theprefence of 

 thefe fmall foreign bodies would influence the refult of the ex- 

 periment in a fenfible manner. 

 A rough furface We ought carefully to diitinguifh thofe furfaces which appear 

 may re tk a as unpo ijft ie( ] to our eves but which in facl are not fo, from thofe 



much light as a \ •> ' 



imocther. which reflect little or no light. 



Metals not lefs It is more than probable that the furface of a metal is always 

 [ofin <ftl ''°' f0r polifhed, and even always equally fo in all the cales wherein 

 lifli. the metal is naked and clear and clean, notwithstanding all the 



mechanical means which may be ufed to fcratch its furface and 



break the glare of its lullre. 

 If the radration Let us return to the comparifon of the evaporation of water 

 of heat defcend f rom the f ur f ace f the earth, with the emiflion of caloric ra- 



on furrounding 



bodies, it will diating from the furface of a heated body, and let us fuppofe 



be of no confe- f or an jnftant,' that the evaporation of the water from the fur- 

 qfaence whether r .. Ai , , , , , , r , , 



the radiating * ace °* tne earth does not depend on the heat ot the earth 



body be policed itfelf, but that it is caufed merely by the influences of fur* 



rounding bodies, as for example, by the rays of light received 



from the fun. It is evident that, in this cafe, the evaporation 



could not be fenfibly greater in a mountainous country than in 



the plain f^and by an eafy analogy we fee, that if hot bodies 



be cooled, not in confequence of the emiffion of fome material 



fub#anue from their furfaces, but by the pofitive action of rays 



fent 



