CAL

CAL

( 193 )

Hence it appears that when a heated ther-mometer furrounded by about an inch in thick-nefs of confined air takes 576 feconds to cool60 degrees of Reaumur (135 0 Fahr.) it will takeI032 feconds to cool to the fame point when16 grains of lint are diffufed through the con-fined air, and 1305 feconds when the fameweight of eider down is ufed; fo that, as theconducing power is inverfely as the times, ifthe power of air be reckoned as 1000, that ofthe lint will be 558, and of the eider down 441.

But lint, eider down, fur, and fimilar floccu-lent fubftances may be confidered as fo manycompounds of fine folid matter, with air in-feparably adhering to them, and filling all theirinterftices, and their true conducting power,iiftindt from that of the air they contain, can-lot be determined by fuch experiments as thefe"bove related.

The conducting power of thefe fubftances isfurther diminifhed by the clofenefs with whichtheir particles are heaped together, at lead to acertain limit, provided this compreffion doesnot proceed to alter their texture, which againwould have a contrary effect. Thus CountRumford found that when the fame fpace whichin the above experiment was occupied by 16grains of eider-down, was filled, firft with 32grains, and then with 64 grains, the times em-ployed in the paffage of the 60 degrees of heatwere fucceffively encreafed from 1304 fecondsto 1472", and 1615".

On the other hand, to fhew the effect ofmere texture, fimilar comparative trials weremade of the conducting power of equal weightsof raw filk, of ravelings of white taffeta, andof common fewing filk, of which the firft hasthe fineft fibre, the fecond lefs fine, and thethird, from being more twitted and harder, ismuch coarfer. The difference between thefethree modifications of the fame fubftance is veryftriking, the raw filk detaining the heat for1284 feconds, the taffeta ravelings 1169", andthe filk thread only 917''.

Hence as a general inference it may beobferved that, as far as the mere conductingpower of animal and vegetable fibre is concernedthe fubftances that detain heat (or cold) thelongeft, that is, that have the leaft conducingpower, or are the ivorfl conductors, are thofe inwhich the fibres are the fineft and the moft.condenfed, provided that this condenfation doesnot amount to that degree which would expelthe interftitia! air. But again, the furface im-mediately in con tael with the outer air, partswith heat largely by radiation, as well as by the

VOL. I.

true conducting power of air, and thereforethe fuperior. power of propelling caloric pof-feffed by unequal and foft fubftances, over thofethat are hard, fmooth, and polifhed, and whichatfts quite in a contrary way to the conductingpower, muft always be taken into confideration.

Of the Conducting Power of Air.

It has been already obferved that when aheated fubftance is freely expofed to the air (orany aeriform medium) it parts with its excefsof heat very largely by radiation, which is moreor lefs intenfe according to the degree of theexcefs of heat, the nature of the furface inimmediate contaCt with air, and the like; butradiation is not the only way in which heat isloft, for much of it paffes off by the conductingor more properly the carrying power of the airitfelf. This is amply proved by the vaft effeCtwhich agitation of the air produces in encreafingthe rapidity of cooling, which is readily ex-plained by fuppofing that the particles of coldair when they come fucceffively in contact with,the heated furface, abforb part of the heat, andthus convey it away into the diftant atmofphere.This kind of conveyance of heat therefore differsboth from radiation, which is propagated to animmenfe diftance in an inftant of time, and fromthe flow and fucceflive mode in which heat isconducted through folids, pervading particleafter particle, and as it were faturating oneftratum of particles before it proceeds to thenext. Here on the other hand (and the famewith liquid mediums) as the heat is conveyedalong with the particles of the air with whichit unites, its motion is precifely that of thefeparticles, falter or flower according to the degreein which it expands them, and the buoyancywhich it thus imparts, and alfo according tothe force of any other propelling power whichthe air may have received. We {hall find there-fore that cool air conveys heat from a hot fur-face in direct proportion both to the rapiditywith which it is renewed, and to its coldnefswhen it ftrikes the heated body.

The cold produced .by fweeping winds is 3matter of common obfervalion, but it is betterto have rec.ourfe to direCt experiment. Thefollowing is eafily performed. Fill a glafs ma-trafs, or any other veffel whatever, with boilingWater, introduce a thermometer, and Amply putit in the body of a blaft furnace (without fire)and obferve the point to which it cools in agiven time. Afterwards repeat the obfervationwith the fame veffel, in the fame fituation,and with no other difference but that of work-ing the bellows during an equal time, and the2 B