THE TANK MODE OF HEATING. 



213 



ing effects of wind upon it are not so great 

 as is generally supposed. Solids differ 

 greatly in their heat-conducting powers. 

 The slow conducting power of such 

 bodies as porcelain, brick, and glass, may 

 be contrasted with the rapid conducting 

 power of some of the metals, by holding 

 one end of a piece of each substance in a 

 flame; the metal will soon become too 

 hot for the hand, while the porcelain may 

 be heated to redness in the flame without 

 its being felt to be much warmer at the 

 other end. 



" When a heated body cools under ordi- 

 nary circumstances, it is by the united 

 effects of radiation and conduction, and 

 the rate of cooling increases considerably, 

 in proportion as the temperature of the 

 heated body is greater than that of the 

 surrounding medium. We have seen 

 that the cooling effect of radiation de- 

 pends greatly on the nature of the sur- 

 face ; but it is a remarkable fact, that the 

 cooling effect of the air by conduction has 

 no reference to the nature of the surface. 

 It is the same on all substances, and in 

 all states of the surface of those sub- 

 stances. The air in contact with such 

 surfaces robs them of a portion of heat, 

 and immediately ascends to make way 

 for other portions of air, which repeat the 

 process. By these two processes, the body 

 cools down to the temperature of the sur- 

 rounding air, the conductive power of 

 which varies with its elasticity or baro- 

 metric pressure ; — the greater the pressure 

 the greater the cooling power. It has also 

 been shown by Dulong and Petit, that the 

 ratio of heat lost by contact of the air 

 alone is constant at all temperatures ; 

 that is, whatever is the ratio between 40° 

 and 80°, is also the ratio between 80° 

 and 160°, or between 100° and 200°. 



" It was long supposed that a certain 

 relation existed between the radiating 

 and conducting powers of heated bodies ; 

 that the variation between them was ex- 

 actly proportional to the simple ratio of 

 the excess of heat; that is, supposing 

 any quantity of heat to be given off in a 

 certain time at a specified difference of 

 temperature, at double that difference 

 twice the quantity of heat would be given 

 off in the same time. This law does, to a 

 certain extent, apply where low tempera- 

 tures are concerned, but does not hold at 

 high temperatures. Thus, in a set of 



experiments by Dulong and Petit, the 

 total cooling at 60°, and 120°, (Centigrade,) 

 was found to be about as 3 to 7 ; at 60° 

 and 180°, as 3 to 13 ; and at 60° and 

 240°, as 3 to 21 : whereas, according to 

 the old theory, these numbers would have 

 been as 3 to 6, 3 to 9, and 3 to 12. When 

 the excess of temperature of the heated 

 body above the surrounding air is as high 

 as 240° Cent., or 432° Fahr., the real 

 velocity of cooling is nearly double what 

 it would have been by the old theory — 

 varying, however, with the surface. Since 

 the heat lost by contact of the air is the 

 same for all bodies, while those which 

 radiate most, or are the worst conductors, 

 give out more heat in the same time than 

 those bodies which radiate least, or are 

 good conductors, it might be supposed 

 that those metals which are the worst 

 conductors would be the best adapted for 

 vessels or pipes for warming by radia- 

 tion. " — Tomlinson. 



" Such would be the case," says Hood, 

 " if the vessels were infinitely thin ; but 

 as this is not possible, the slow conduct- 

 ing power of the metal (iron) opposes an 

 insuperable obstacle to the rapid cooling 

 of any liquid contained within it, by pre- 

 venting the exterior surface from reach- 

 ing so high a temperature as would that of 

 a more perfectly conducting metal under 

 similar circumstances ; — thus preventing 

 the loss of heat both by contact of the 

 air and by radiation ; the effect of both 

 being proportional to the excess of heat 

 of the exterior surface of the heated body. 

 If a leaden vessel were infinitely thin, the 

 liquid contained in it would cool sooner 

 than in a similar vessel of copper, brass, 

 or iron ; but the greater the thick- 

 ness of the metal, the more apparent be- 

 comes the deviation from this rule ; and 

 as the vessels for containing water must 

 always have some considerable thickness, 

 those metals which are the worst con- 

 ductors will oppose the greatest resistance 

 to the cooling of the contained liquid." 



§ 5. — HEATING BY HOT-AIR STOVES. 



It is now upwards of forty years since 

 we first heard the sentence of condem- 

 nation pronounced on hot-air stoves as 

 applied to hothouse-heating ; and many 

 will recollect the mild but expressive 



