230 



HEAT. 



thermometer with the substance to be tested, and finding its rate of cool- 

 ing for different excesses of temperature above the surroundings. It 

 was then exposed to radiation from a source, and the steady state was 

 attained. The heat lost per second was proportional to the rate of cool- 

 ing previously found for the temperature now observed, and the loss was 

 equal to the heat gained by absorption. The results obtained agreed in 

 their general character with those of Melloni. 



Comparison of Reflecting Powers. The earliest work on this 



subject was done by Leslie. He employed an arrangement represented 

 in Fig. 135. was a Leslie cube filled with boiling water and with a 

 blackened face turned towards a tin spherical mirror M. The focus 

 conjugate to C was /, but the rays were intercepted on their way 

 thither by a plate of the substance to be tested placed at ab. The rays 

 came to a focus at /', and at that point was one bulb of a differential 

 thermometer. The rise in temperature given by the thermometer was 



FIG. 135. Leslie's Comparison of Reflecting Powers. 



proportional to the reflecting power. The following are some of his 

 results : 



Reflecting power of surfaces for radiation from a black cube at 100 C. 

 in terms of brass taken as 100 



Brass . 



Silver 



Tin . 



Steel 



Glass 



Lampblack 



100 

 90 

 80 

 70 

 10 

 



Lampblack practically reflected none of the radiation falling on it. 



Melloni obtained results generally confirming those of Leslie. But 

 we know that the reflecting power for light varies with the quality of the 

 incident light and also with the particular specimen of the substance 

 tested. And there is no doubt that there is a like variation in the case 

 of invisible radiation. We should expect the results of different observers 

 to differ, and they do differ. 



As illustrating this we give the following results of De la Provostaye 

 and Desains obtained from a Locatelli lamp as source. It will be seen 

 that the order is slightly different from that found by Leslie : 



