CHAMBERS'S INFORMATION FOR THE PEOPLE. 



PROPAGATION OF HEAT. 



Heat is transferred from one portion of matter 

 to another in three different ways, which are 

 termed conduction, convection, and radiation. 

 Conduction implies the passage of heat from one 

 particle of matter to another in physical contact 

 with it. Convection is the conveying or carrying 

 of heat by particles of matter raised in tempera- 

 ture, and set in motion. Radiation is the emission 

 of heat by a body such, for example, as a mass of 

 red-hot iron at rest, and not in physical contact 

 with the substances to which it communicates 

 heat. The heat passes in radii, or rays, like those 

 of sunlight, which can find their way even through 

 a vacuum, and do not appear to require the assist- 

 ance of ponderable matter to determine their 

 transference. 



Conduction. 



Conduction is best seen in solids, and particu- 

 larly in metals, which are the best conductors. A 

 rod of iron placed with one extremity in the fire 

 speedily becomes hot at the opposite extremity, 

 owing to the conduction of heat from particle to 

 particle along the rod. Dense bodies are gener- 

 ally the best conductors ; light and porous ones 

 the worst ; but this rule does not always hold. 

 Feathers, down, fur, flannel, and most of the 

 fabrics used for winter dresses, owe their so-called 

 warmth to their low-conducting power for heat. 

 Their action is altogether negative, being limited 

 to the prevention of the rapid escape of heat 

 generated by the living beings whose bodies they 

 cover. Hence the best way to preserve ice is to 

 wrap it in flannel, or other so-called warm cover- 

 ing ; for the same means that retard the escape 

 of heat from the living body, retard the access of 

 heat from the air and surrounding objects to the 

 ice. 



The relative conductivities of the principal 

 metals are exhibited in the following table : 



On the same scale, the conductivity of marble 

 would be expressed by -8, of porcelain by -6, of 

 brick earth by -55. Wood conducts fully twice 

 as well in the direction of the fibre as transversely. 



Vessels of porcelain and glass are liable to 

 crack when hot water is suddenly poured into 

 them, because these substances conduct heat 

 slowly, and thus the part first touched by the 

 water becomes expanded, while the parts adjoining 

 are yet cold and unexpanded. 



Liquids and gases are very bad conductors of 

 heat, although, from the rapidity with which they 

 rise in temperature, when heat is applied to them, 

 they appear to be among the best conductors. 



Convection. 



Liquids and gases rise in temperature chiefly 

 in consequence of the convection, not the con- 

 duction of hear by their particles. If a spirit-lamp 

 is applied to the top of a tube filled with water, 

 M 



the upper portion of the liquid is soon heated to 

 boiling, while hours will elapse before even a 

 slight degree of heat will reach any distance down 

 the column. But if the lamp is applied at the 

 bottom of the tube, the heat is soon felt at the 

 top, and the whole liquid is made to boil in a few 

 minutes. This remarkable difference is owing 

 to a motion that takes 

 place among the par- 

 ticles of the liquid. The 

 portions resting on the 

 bottom, being expanded 

 and made lighter by the 

 heat, begin to ascend, 

 and thus circulating 

 currents are established 

 which convey the heat 

 to all parts of the mass. 

 This circulation may 

 be made visible in a 

 glass flask (fig. 20) 

 nearly filled with water, 

 having a few bits of 

 blue litmus swimming 

 in it, and with a spirit- 

 lamp below. 



Air and other gases 

 are raised in tempera- 

 ture in the same way 

 that liquids are. They Fig. 20. 



conduct heat with ex- 

 treme slowness, as may be proved by applying 

 heat to the top of an air-tight glass vessel with a 

 thermometer suspended a little below the heated 

 portion. But when the heat is applied from below, 

 currents of circulation immediately begin, as in 

 the case of a liquid. 



Our sensations lead us into many errors regard- 

 ing the temperature of bodies, which attention to 

 their different conducting and conveying powers 

 enables us to correct. Thus, of the several articles 

 in the same room, the table feels colder than the 

 cloth that covers it, and a marble slab colder than 

 the table. They are, in fact, all of the same tem- 

 perature the temperature, namely, of the air in 

 the room ; but the marble being the best con- 

 ductor, robs the hand most quickly of its heat. 



The cooling effect of a liquid or fluid is much 

 increased by motion, as when the hand is moved 

 through cold water, thus bringing fresh unwarmed 

 portions in contact with it in rapid succession. It 

 is on this last principle that a degree of cold may 

 be borne with a calm atmosphere, which becomes 

 quite killing with even a moderate wind. When 

 air is confined and prevented from circulating, it 

 forms a non-conductor and a warm protection. 

 Hence the effect of double-windows, and of plaster 

 put on laths, so as to inclose a plate of air be- 

 tween it and the wall, instead of being laid on 

 the wall itself. Woolly coverings and furs imprison 

 the air within their substance, and prevent it from 

 circulating, while they afford but few points of 

 solid contact for the direct conduction of heat. 

 These two circumstances combined, give them 

 their remarkable power in arresting the escape of 

 heat ; and that power is greater the finer and 

 lighter their texture. Swan's-down is said to be 

 the most perfect insulator of heat. From the 

 same causes, snow is an excellent non-conductor, 

 and, like a fleece of wool, protects the earth from 

 ! any cold much below 32. 



