NATURAL PHILOSOPHY MATTER, MOTION, AND HEAT. 



Radiation. 



Every hot body in the act of cooling, besides 

 losing heat by the conductive and convective 

 action of the solids and fluids in its neighbour- 

 hood, parts with much heat by radiation. Rays 

 of heat pass away from the hot body till it has 

 reached the temperature of the air or surrounding 

 medium. In proof of this, it is only necessary to 

 hang a hot body in the vacuum of the air-pump, 

 when it rapidly cools, although it does not lose 

 heat by either conduction or convection. 



The rate of cooling of a hot solid body, so far 

 as radiation is concerned, is remarkably influenced 

 by the state of its surface, and, in the case of 

 liquids and gases, by the state of the surface of 

 the vessels containing them. Thus, hot water 

 placed in a tin vessel coated externally with lamp- 

 black, cools twice as fast as it does in a bright tin 

 vessel. From these observations, it appears that 

 a kettle covered with soot is much less suited for 

 retaining water warm, than if it had a polished 

 metallic surface. So, also, metallic tea-pots and 

 coffee-pots are preferable to those of porcelain and 

 stoneware. 



Absorption and reflection of Rays of Heat. Rays 

 of heat follow almost the same laws as to reflec- 

 tion, absorption, refraction, &c. as rays of light. 

 In fact, radiant heat is manifestly identical with 

 light, differing from red light, for instance, as red 

 light differs from blue that is, by having longer 

 waves (see SOUND AND LIGHT). When heat-rays 

 fall on the surface of a body, they either enter it 

 or are reflected. Those, again, that enter are either 

 transmitted, like light through glass, or are re- 

 tained and absorbed. It is only the rays that are 

 absorbed that warm the body ; those that are 

 reflected off, as well as those that are transmitted, 

 produce no effect on the temperature. 



Surfaces that radiate heat best, are found also 

 to imbibe it most readily. If a table, then, is 

 formed of substances according to their power of 

 radiating heat, the same table will serve for their 

 power of absorption. The following is such a 

 table, the radiating and absorbing power of a 

 surface of lampblack being expressed by 100 : 



Lampblack. loo 



White-lead 100 



Writing-paper 98 



Glass 90 



Polished Iron 23 



Mercury 23 



Tin 14 



Brass, polished 7 



Copper 7 



Gold 3 



Silver 3 



Since all the rays not absorbed by a surface must 

 be reflected, this table, read from the bottom up- 

 wards, will give the same surfaces in the order of 

 their reflecting powers. It thus appears that the 

 best protection for the head from the burning rays 

 of the sun is a polished metal helmet 



Colour has comparatively little influence on the 

 radiation or absorption of heat so far as obscure 

 or non-luminous rays are concerned. In the 

 above table, lampblack and white-lead rank equal ; 

 and two garments of the same material, the one 

 white and the other black, dissipate the heat of 

 the body at the same rate, and afford the same 

 protection from the rays of a hot stove. But in 

 regard to luminous heat, the case is different A 

 beam of the sun contains both invisible and 

 visible rays the former, however, being much 

 greater in amount than the latter. Now, when 

 such a beam falls on two pieces of cloth, one 



black and the other white, the effect of the obscure 

 rays is the same on both ; but the black surface 

 absorbs also the luminous rays, while the white 

 rejects them. The black is thus more heated than 

 the white. As a protection against the direct rays 

 of the sun, light-coloured clothing is thus better 

 than dark. 



All bodies, even the coldest, are constantly 

 radiating off more or less heat, according to their 

 temperature; all are therefore both giving and 

 receiving rays ; but the warmer give more than 

 they receive, the colder receive more than they 

 give. A surface presented towards the open sky, 

 with nothing to radiate or throw back the rays it 

 is emitting, soon becomes cold ; but the slightest 

 curtain, such as a net, hung up before it, sensibly 

 arrests the dissipation of heat. It is in this way 

 that clouds act as warm curtains to the earth, and 

 often prevent frosts in spring and autumn nights. 

 The different radiating powers of bodies explains 

 why dew is sooner deposited on some substances 

 than on others. Those that are good radiators 

 lose their heat most quickly, and thus condense 

 the vapour of the atmosphere. 



Transmission of Thermal Rays. Some sub- 

 stances, it has been observed, allow heat to pass 

 through them, as light passes through glass ; such 

 substances are called diathermanous. Bodies are 

 not diathermanous and transparent in the same 

 degree ; for black glass transmits heat well, and 

 water, which is highly transparent, is the least 

 diathermanous of liquids. Of solid bodies, rock- 

 salt is the most diathermanous, alum the least so. 

 The powers of gases and vapours to transmit 

 radiant heat have recently been investigated by 

 Professor Tyndall, with very striking results. The 

 most important are those concerning the com- 

 ponent elements of our atmosphere. Oxygen, 

 nitrogen, and hydrogen transmit almost perfectly, 

 and so does pure air, which is a mixture of oxygen 

 and nitrogen. All the other gases absorb more or 

 less of the rays. If the absorption of air is stated 

 as i, that of carbonic acid is 90, and of ammonia 

 is 1195. But this is nothing to the absorbing 

 power of the vapour of water. When the atmos- 

 phere is in an average state of humidity, the 

 invisible aqueous vapour contained in it absorbs 

 72 times as much radiant heat as the air itself, 

 although the quantity of the latter is 200 times 

 that of the former. The consequences of this are 

 of vital importance to the inhabitants of our globe. 

 If the atmosphere were without vapour, the sun's 

 rays, both luminous and obscure, would pass 

 through it without warming it, and without losing 

 any of their power, and would scorch the earth's 

 surface while exposed to them ; while in the 

 absence of the sun, the radiation from the earth 

 would escape without obstruction, and rapidly 

 produce excessive cold. This effect is partially 

 experienced in elevated positions, where the 

 vapour is comparatively rare ; on lofty mountains, 

 the direct rays of the sun are intensely hot, while 

 the air is cold. The vapour of the air thus acts 

 as a blanket to the earth, tempering the direct 

 rays of the sun, and again intercepting the earth's 

 radiation. 



Glass transmits luminous rays, but is atherman- 

 ous to obscure rays ; hence, a glass frame, in 

 gardening, has been called ' a trap to catch sun- 

 beams.' The light of the sun passes through the 

 glass, and is absorbed by the earth as heat ; but 



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