418 



RADIATION OF HEAT. 



the reflector, so that the rays of heat take a direction perpendicular to the ra- 

 diating surface ; but if each point of the surface radiates heat in all possible 

 directions, it will follow that the surface, when presented obliquely to the mir- 

 ror, will still affect the thermometer. When the surface of the canister was 

 presented thus obliquely, the effect produced on a thermometer was found to 

 be the same as would be produced by a surface of less magnitude, in the pro- 

 portion of the actual magnitude of the radiating surface to that of its projection. 

 It follows, therefore, that the more inclined the radiating surface is to the di- 

 rection of the radiation, the less will be the intensity of the radiation ; but in 

 general this intensity will be diminished, in the proportion of the actual magni- 

 tude of the radiating surface and the magnitude of its orthographical projection 

 on the mirror. 



We have hitherto supposed the nature of the radiating surface to remain un- 

 altered. The effect of any change in this, however, may be easily ascertained 

 by covering the side of the canister with the different substances the effect of 

 which is required. Thus, let the four sides of the canister be coated with dif- 

 ferent substances one with lampblack, another with isinglass, another with 

 china ink, and a fourth left uncovered, and therefore presenting a surface of 

 polished tin. The vessel being now filled with hot water, all the surfaces will 

 acquire the same temperature, and may be successively presented to the re- 

 flector at the same distance ; they will be observed to produce different effects 

 on the thermometer. If the lampblack depresses the liquid 100, the china 

 ink will depress it 88, the isinglass 80, and the tin 12. The great differ- 

 ence in the radiating power produced by the different nature of the surfaces 

 will be hence very apparent. 



The inquiries of Professor Leslie were directed to this point with great ef- 

 fect, and he found that various substances possessed very different radiating 

 powers. In general, metallic bodies proved to be the most feeble radiators. 

 The following table exhibits the relative power of radiation of different sub- 

 stances, as exhibited in these experiments : 



Lampblack 100 



Water, by estimate ' 100 



"Writing-paper 98 



Rosin " 96 



Sealing-wax 95 



Crown glass 90 



China ink 88 



Ice 85 



Minium ... 80 



Isinglass 80 



Plumbago 75 



Tarnished lead 45 



Mercury 20 



Clean lead 19 



Iron polished 15 



Tin-plate 12 



Gold, silver, copper 12 



When the substance forming the radiating surface remains of the same na- 

 ture, its radiating power is subject to considerable elevation, according to its 

 state with respect to smoothness, or roughness. In general, the more polished 

 and smooth a surface is, the more feeble will be its power of radiation. Any- 

 thing which tarnishes the surface of metal also increases its radiating power. 

 In the preceding table, tarnished lead radiated 45, while clean lead radioed 

 only 19. If the surface of a body be rendered rough by mechanical means, 

 such as scratching with a file, or with sand-paper, the radiating power is in- 

 creased. 



Leslie also proved that the particles forming the surface of a body are not 

 the only ones which radiate, but that radiation proceeds from particles at a cer- 

 tain small depth within the surface. He determined this curious point by cov- 

 ering one side of a vessel containing hot water with a thin coating of jelly, and 

 pulling on another side four times the quantity. In each case, when dried, the 



