

137 



The thiid way in which heat U communicated from one body 

 to another U by moans of / In conduction and con- 



vection the particles of matter to be heated wore brought into 

 close contact with the source of heat : we shall now find, 

 however, that heat can pass from one body to another without 

 i.-tu ii . i.nt.iot, and even without altering the temperature of 

 the medium through which it passes. A striking illustration 

 of tint l:iit<T f.ict is seen in the experiment of netting light 

 moos by condonwing the sun's rays on them 

 through a Ions of ioe. The heat passes through it in sufficient 

 quantities to inflame the sub- 

 stances, and yet the ioe is 

 unlimited. 



When we stand a little dis- 

 tance from a fire we at once ex- 

 perience a sensation of warmth, 

 no particles of matter appear to 

 pass, and yet the influence of 

 the fire is felt. Rays of heat are 

 given off by the burning fuel, 

 which create in us the feeling of 



warmth. The presence of the air is evidently not necessary for 

 their passage, since we experience the heat of the sun, whose 

 rays must pass through space. We may also prove this fact 

 experimentally by letting two charcoal points connected with a 

 powerful battery touch under an exhausted receiver. Bays of 

 heat will be given off despite the absence of the air, and their 

 presence will at once be felt. 



Now we find that radiant heat obeys the same laws an light 

 does, the rays being given off in all directions, and, in a uniform 

 medium, always travelling in a straight line. This may easily 

 be shown by suspending a heated body in the air, and then 

 holding a thermo-electric pile at equal distances on each side 

 of it. If, however, a plate of metal be interposed between the 

 pile and the source of hoat, the rays will at once be intercepted, 

 and the needle will return to zero. The power of radiant heat 

 diminishes, as in the case of light, with the square of the dis- 

 tanco. 



If we take a heated body, such, as a cubical vessel, M, filled 

 with boiling water (Fig. 28), 

 and place it in front of a 

 concave mirror, we shall nnd 

 that the rays of heat are re- 

 flected from its surface, in 

 the same way as those of 

 light are. Let a differential 

 thermometer be placed in the 

 focus of the mirror, a screen, 

 A, being placed so as to keep 

 off the direct rays from M. 

 The indicating bubble will at 

 once show the increase of 

 temperature ; if the bulb be 

 moved at all out of the focus, 

 the bubble will return to its 

 place, clearly showing that 

 the rays have been reflected 

 and brought to a focus. By 

 means of a small mirror we 

 can easily prove that in the 

 case of reflected heat the 

 angle of incidence is always 

 equal to the angle of reflec- 

 tion. An ordinary sheet of 



tin held in front of a fire will illustrate this reflection of heat, 

 and from it wo shall understand the use of reflectors in roasting. 

 As the amount of heat reflected depends upon the brightness of 

 the reflectors, the necessity for keeping them clean and bright 

 will be apparent. For the same reason, the back and sides of a 

 stove should be kept as clean as possible, so as to throw out 

 the heat into the room. 



Rays of heat may be refracted as well as reflected. When a 

 oeam from an electric lamp is caused to fall upon a prism, the 

 luminous rays are bent out of their course, and resolved into the 

 prismatic colours ; the heat-rays are likewise diverted ; and if we 

 place behind the spectrum a metal screen with a narrow slit in 

 it, so as only to allow the rays from one part of the spectrum to 

 pass at a time, we may, by a pile, test the heat of different 



parti. In doing so we find that at the violet end of (he 



i rum there w but little heat; even in the yellow, though 



that ia the moat luminous part, there in not much. At the red 



| portion the heat a greater, but iU intensity is greatest when 



the pile ia moved altogether beyond the visible spectrum, so 



that tlin most intent*) portion of the beat is altogether non- 



| luminous. The thermal spectrum, in fact, overlap* (he visible 



one. 



U'iicn we commence to try experiments on the radiat 

 heat, we soon find that different surfaces possess different 



powers of throwing off rays of 

 heat. This is easily shown by 

 means of a " Leslie " cube (fig. 

 29), which consists simply of a 

 tin or pewter cube with an 

 opening on one side, by which 

 it can be filled with boiling 

 water. One side may be covered 

 with a layer of gold-leaf, another 

 with glass, a third with lamp- 

 black, while the fourth is left 



blank. Each side is now turned in succession towards the 

 thermo-electric pile, and the exact deflection of the needle 

 noted. Other substances may then be laid on the sides of the 

 cube, and in this way a table showing the radiating power of 

 different bodies may be drawn up. 



When the gilded face is towards the pile, little effect will 

 be produced ; if the pewter be a little tarnished, a greater 

 deflection will be produced when that side is turned to the pile. 

 When the glass side is presented, the intensity will be much 

 more, while with the lamp-black it will be most of all. A* 

 lamp-black is the best radiator, its power is represented by 100, 

 and then the power of gold and other brilliant metals will be- 

 between 12 and 15. 



Another way in which we may show these different powers of 



radiation is to observe the time which water takes to cool when 



placed in different vessels. Take, for example, two similar 



i cubes, and let one be covered with lamp-black while the other 



ia left bright. Fill both with boiling water, and after some 



time test the temperature 

 of each. That coated with 

 lamp-black will be found 

 several degrees cooler than 

 the other. It has radiated 

 heat more rapidly, and hence 

 has lost a larger amount. 



If we substitute a lump of 

 ice or a cube of ice-cold 

 water for the vessel M (Fig. 

 28), and place the thermo- 

 meter as before, it will fall, 

 and thus indicate an ap- 

 parent radiation of cold. 

 This is only apparent, how- 

 ever ; both the ice and the 

 thermometer possess a cer- 

 tain amount of heat, which 

 they radiate. The thermo- 

 meter, however, being at a 

 higher temperature, throws 

 off more intense rays, and 

 hence, as it parts with more- 

 heat than it receives, itt- 

 temperature falls. The chill 



felt when standing near a cold surface may be similarly explained. 

 When rays of heat fall upon any substance, they are divided 

 into three parts. One portion is reflected from the surface, 

 according to the laws already mentioned ; a second part is 

 irregularly scattered, and is known as diffused heat. This cor- 

 responds to the light which is irregularly reflected from any 

 substance, and renders it visible. The third portion is absorbed 

 by the substance, and raises its temperature. When a number 

 of surfaces are exposed thus to the rays from a heated body, 

 their absorbing powers will be found to differ very greatly, in 

 some cases nearly all the heat being absorbed, while in others 

 by far the greater portion is reflected. These two amounts 

 will, as a rule, be inversely proportional, the best reflectors 

 being the worst absorbers, and vice versd. 



