HEAT 



meter. Placing the pile out of the line from a 

 source of heat to an aperture in a screen, no effect 

 is observed ; but deflection of the needle at once 

 occurs when the pile is placed in the line which 

 light would have followed if substituted for the 

 heat. 



A concave mirror, which would bring rays of 

 light proceeding from a given point to a focus at 

 another given point, does the same with heat, the 

 hot body being substituted for the luminous one, 

 and the pile placed at the focus. Heat, then, is 

 reflected according to the same laws as light. A 

 burning lens gives a capital proof of the sun's heat 

 and light being subject to the same laws of refrac- 

 tion. When the solar Spectrum (q.v.) is formed 

 by means of a prism of rock-salt ( the reasons for 

 the choice of this material will afterwards appear), 

 the thermo-electric pile proves the existence of 

 heat in all the coloured spaces, increasing, how- 

 ever, down to the red end of the spectrum, and 

 attaining its maximum beyond the visible light, 

 just as if radiant heat were (as it must be) light 

 with longer waves. 



Some bodies, as glass, water, &c., transmit, 

 when in thin plates, most of the light which falls 

 on them ; others, as wood, metal, coloured glass, 

 &c., transmit none or little. A plate of rock-salt, 

 half an inch thick, transmits 96 per cent, of the 

 rays of heat which fall on it ; while glass, even of 

 a thickness of one-tenth of an inch, transmits very 

 little. In this sense, rock-salt is said to be diather- 

 manous, while glass is said to be adiathermanous, 

 or only partially diathermanous. Most of the 

 simple gases, such as oxygen, hydrogen, &c., and 

 mixtures of these, such as air, oppose very little 

 resistance to the passage of radiant heat ; but the 

 reverse is in general the case with compound gases. 

 It has recently been asserted that water-vapour in 

 particular is exceedingly adiathermanous. The 

 question is one of very considerable difficulty, 

 owing to the fact that it is almost impossible to 

 experiment upon vapour alone. The presence of 

 dust particles always produces deposition of water, 

 which is a very good absorber of radiant heat. 



But there are other remarkable phenomena of 

 radiant heat which are easily observed, and which 

 have their analogy in the case of light. ( 1 ) Un- 

 stained glass seems equally transparent to all kinds 

 of light. Such is the case with rock-salt and heat. 

 (2) Light which has passed through a blue glass 

 ( for instance ) loses far less per cent, when it passes 

 through a second plate of blue glass. Similarly 

 heat loses (say) 75 per cent, in passing through one 

 plate of crown-glass, and only 10 per cent, of the 

 remainder (say) in passing through a second. (3) 

 Blue light passes easily through a blue glass, which 

 almost entirely arrests red light. So dark heat 

 passes far less easily through glass than bright heat 

 does. These analogies, mostly due to Melloni, are 

 very remarkable. 



Again, light can be doubly refracted, plane 

 polarised, circularly polarised. All these pro- 

 perties have been found in radiant heat by Prin- 

 cipal Forbes. 



The beautiful investigations of Stokes, Balfour 

 Stewart, and Kirchhoff have shown us that bodies 

 which most easily absorb light of a particular 

 colour give off most freely, when heated, light of 

 that colour ; and it is easily shown by experiment 

 that those surfaces which absorb heat most readily 

 also radiate it most readily. Thus, it was found 

 by Leslie that when a tinned-iron cube full of boil- 

 ing water had one side polished, another rough- 

 ened, a third covered with lampblack, &c., the 

 polished side radiated little heat, the roughened 

 more, while the blackened side radiated a very 

 great quantity indeed. And again, that if we 

 have ( say ) three similar thermometers, and if the 



bulbs be ( 1 ) gilded, ( 2 ) covered with roughened 1 

 metal, (3) smoked, and all be exposed to the same 

 radiation of heat, their sensibility will be in the 

 order 3, 2, 1. A practical illustration of this is 

 seen in the fact that a blackened kettle is that in 

 which water is most speedily made to boil, while a 

 polished one keeps the water longest warm when 

 removed from the fire. Again, if a willow-pattern 

 plate be heated white-hot in the fire, and then 

 examined in a dark room, the pattern will be 

 reversed a white pattern being seen on a dark 

 ground. It is this law of equality of radiating and 

 absorbing powers that mainly gives rise to the 

 superior comfort of white clothing to black in 

 winter as well as in summer ; radiating less in 

 winter, it absorbs less in summer. 



Much has been argued about the separate exist- 

 ence of cold, from such facts as these : A piece of 

 ice held before the thermo-electric pile produces an 

 opposite deflection of the galvanometer to that pro- 

 duced by a hot ball. If a freezing mixture be 

 placed at one focus of a spheroidal mirror, and a 

 thermometer with a blackened bulb at the conju- 

 gate focus, the latter will fall speedily, though 

 very far off from the mixture. Now, the true 

 explanation of such observations is to be found in 

 what is called the 'Theory of Exchanges,' first 

 enunciated by Prevost, and since greatly extended 

 and carefully verified by Stewart, which is to this 

 effect : ' Every body is continually radiating heat 

 in all directions, the amount radiated being greater 

 as the temperature is higher.' Thus the radiation 

 from a body depends on itself alone, the amount 

 absorbed depends on the radiation which reaches it. 

 Hence the apparent radiation of cold in the experi- 

 ments above mentioned is due to the fact of the 

 pile or thermometer radiating off more heat than it 

 receives, as its temperature is higher than that of 

 the freezing mixture to which it is opposed. From 

 this it is evident that any number of bodies left 

 near each other tend gradually to assume a com- 

 mon temperature. By this theory of exchange* 

 we explain the cold felt in sitting opposite an 

 open window in a frosty day, even when tnere is no 

 draught. 



Convection. A hot body cools faster in a current 

 of air than in a still 

 atmosphere of the same 

 temperature, evidently be- 

 cause fresh supplies of the 

 colder air are continu- 

 ally brought into contact 

 with it. This carrying 

 off of its heat by a stream 

 of air is an example of 

 convection. It is by con- 

 vection mainly that heat 

 is conveyed throughout 



liquids and gases. Thus, 

 when a lamp is applied 

 to the bottom of a vessel 

 of water the heat does not 

 diffuse itself in the water 

 as it would (by conduc- 

 tion) in a mass of metal, 

 but the expansion of the 

 heated water at the bot- 

 tom rendering it lighter, 

 bulk for bulk, than the 

 superincumbent fluid, 



causes it to rise to the surface ; and thus, by con- 

 vection, the heat is diffused through the mass. 

 Conduction, properly so called, can scarcely be 

 shown, though it really exist, in liquids or gases, 

 on this account. The tremulous appearance of any 

 object as seen by light which' passes near a hot 

 surface, as that of a boiler or a red-hot poker, is 

 due to the convection of heat in the air, the warm 



