Pi-odunini and Confinement of Heat, Clothing, &c. 163 



which pafles off in flame and heated vapour. It feems probable that it may vary according to 

 the volatility of the combuftible matter and other circumftances ; but, in general, it feems 

 that the quantity of the former is much lefs than that of the latter. Open fires warm apart- 

 ments by their radiant heat only, all the heat of the vapour being entirely loft up the chimney. 

 It is a remarlcabie fa6l, that radiant heat is emitted in great abundance by all bodies which are 

 capable of ignition, whether they be folid or fluid, combuftible or incombuftible. 



The means of confining heat and directing its operations, conftitute the obje<3: of the third 

 chapter. It is well known that heat pafles more fpeedily through fome bodies than others. The 

 ufeful applications of this property are feen in a variety of inftances ; among which, the wooden 

 handle of a tea-pot, and the cloth made ufe of in handling hot iron, are familiar examples. 

 The Author mentions fome others, and remarks that the conducing powers of no two 

 bodies, with regard to heat, are exactly alike. 



To confine heat, is nothing more than to prevent its efcape out of the heated body, by 

 furrounding that body with a covering, through which it cannot readily pafs. If a covering 

 could be found perfectly impervious to heat, there feems no reafon why the heated body, thus 

 defended, fliould not continue hot for ever; but it is not probable that any fuch body exifts. 



All metals are remarkably good condudtors. Wood, and, in general, all light fpongy bodies, 

 or fuch as contain fluids in their interftices, are bad condudtors *. Mercury, water, and all 

 fluids, are conducSors by circulation, but not perceptibly fo in any other way. Air and the 

 elaftic fluids are very flow condu(Slors, even when permitted to circulate; probably on account 

 of their very fmall mafs: pulverifed bodfes condu6l worfe than the fame bodies in the aggreo-ate. 

 Very dry powder of charcoal is one of the beft we know ; but common air is the fubftance 

 employed by nature to confine heat, and is certainly the beft which can be ufed. 



The warmth of the fur of beafts, is undoubtedly owing to the air entangled or confined in 

 their interftices f. Double windows and walls, fo ufeful in cold countries to confine heat, and 

 in hot climates to prevent its admiflion, operate in the fame way J. The Author proceeds to 

 ■give a ftiort account of his experiments formerly made on thefe fubjefts, and (hews by a 

 curious experiment, that fteam is not a condudior of heat in any refpedl, except by the 

 inteftine motion of it parts. The fame experiment is applied to iliew that heated air and 

 vapour, are alfo non-coqdu(5lors, and may be ufed with the greateft advantage in our operations 

 for confining heat. 



Thefe interefting fpeculations lead in the fourth chapter to an enquiry how the heat is com- 

 municated from flame to other bodies. If the conducing quality of bodies be not altered by any 

 increafe of temperature which does not change their chemical combination, it will be proper to 

 confider flame as a hot wind. It is neceflary, in order that a fluid fliould receive or impart 

 heat, that all its parts fliould feverally come into conta£t wfith the body which gives or receives. 

 Hence a hot body immerfed in air is not cooled except fo far as the air has motion ; and the 



• Philofoph'.cal Journal. 1. 189-295 ' 



f PhilofophicalTranfailiuns, 1792. 



X Double walls and windows are fcarcely ufed in hot covintries. I have fsea light double ftruftures of bamboo 

 ia Indk. N. 



