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'Where the temperature of the heated body ismuch above that of the cold medium, a fimiiareftedt is produced to that of agitating the air,that is, in tending to encreafe and at the fametime to equalize the rate of cooling of the oppo-fite furfaces of metal and paint. In fact anactual encreafe of agitation in the air is therebyproduced, fince the operation of any hot bodymu If be to caufe a conffant afcending currentof hot, and therefore light, air, in proportion asit receives caloric by contact with or proximityto the fource of heat; confequently, the greaterthe degree of pofitive heat, and the more rapidwill be the current of air.

It is alfo obvious that the fame effedf of en-creafing the fucceffive contacts of air is producedby agitating the heated body in Hill air, as byagitating the air again ft the heated body whenat reft. Hence the common practice of batten-ing the cooling of any fubftance by waving itbackwards and forwards, and it appears thatthe refrigerant power of air is always (othercircumftances being the fame) in exadt propor-tion to its velocity.

The conducing power of air is diminiflred byrarefadtion, fo that whatever has a tendency to■dilate air, proportionally leflens its power ofconveying caloric. This diminution proceedsregularly till a compleat vacuum is formed,which, as already mentioned, is a ftill worfeconductor of heat than any kind of air, howeverdilated. Every fubftance which when convertedto vapor produces a dilatation of the air withwhich it combines, alfo diminifhes its conduct-ing power. Thus alcohol, ether, and evenaqueous vapor diffufed in air render it a worfeconductor than before. On the other hand, acondenfation of air obvioufly encreafes its con-ducting power. It alfo appears that many ofthe permanent gaffes have their peculiar con-ducting power, w independent of extraneousvapour or dilatation.

Much remains to be done on the fubject ofthe propagation of caloric through gafeous bodies,and the experiments neceffarv to purfue thisinquiry are complex and difficult. At leaftfour diftinCt calorific actions muft be prefent inexperiments of this nature, and the diftindtpower of each muft be afcertained before anyaccurate conclufions can be drawn. One ofthefe is the radiation of caloric from every heatedfurface when in contact with air; and as in allexperiments veffels muft be employed to confinethe air examined, and thermometers to eftimatethe paflage of heat, the radiations from thefe

folid bodies, and the propul five energy belong-ing to the nature of their furfaces mult be fepa-rately confidered. Another circumftance to heconfidered is the denfity of the air examined,and its degree of moifture, both of which affectthe paffage of heat. A third is the motion of theparticles of air produced, even in clofe veffels,by the partial application of beat, caufmg, asalready explained, an afcending current overthe heated body, of more or lefs velocity accord-ing to the difference of temperature between itand the furrounding medium. Befides thefe,it is probable that air has a proper conducingpower diftinCt from its conveying power; thatis, that even if its particles could be kept atreft on the partial application of heat, caloricwould pais through its fubftance, as it doesthrough a folid, and through every natural bodyhitherto known. But this diftincfion betweenthe conducting and the conveying power willbe more clearly pointed out in the followingfedtion.

Of the Conducting Power of lf r iter.

In examining the propagation of caloricthrough water, radiation is totally out of thequeftion, it having been fully proved that heatdoes not radiate through a denfe liquid medium.Neither can the circumftance of rarity or denfityproduced by external prefl'ure fenfibly affect aninelaftic fluid; but only the change in fpecificgravity produced by the Ample addition or ab-ftraction of caloric. The enquiry is thereforemuch fimpler than that of the paflage of caloricthrough an aeriform medium, and is principallyconfined to thefe two objects, namely, the ca-lorie which is conveyed through a mafs of waterby the afeent of the hotter particles, and theconfequent defeent of the colder ones; and thecaloric which is conducted through water in anydiredtion, and principally downwards, indepen-dant of the internal motions of its particles.

A number of facts of daily occurrence Ihewthe almoft impoflibility of heating water or anyother fluid effectually by applying heat on thefurface, and the comparative eafe with whichit receives heat from the bottom. Thus it muftoften have occurred to chemifts to obferve inthe procefs of diftillation with a ftill and worm-pipe paffmg downwards through a tub of waterin a fpiral direction, that the upper part of thewater which is in contact with the liotteft por-tion of the pipe foon becomes icalding hot andcontinues fo for a long time, whilft the lowerpart of the fame water remains cold for manyhours. But this principle was firft more diftinctly

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