226 HEAT. 



absorbing energy. For when they are in a steady state they are giving 

 out as much as they receive. Now, with the small excess of temperature 

 produced by the radiation, the amount given out is proportional to the 

 excess, and therefore the excess is proportional to the amount received. 

 It is just the same whether the energy was blue light, yellow light, or 

 ultra-red radiation before it was absorbed and converted into heat. If the 

 total energy absorbed is the same the indication of the instrument is the 

 same. When we wish to study wave-length as well as quantity we must 

 first sort the radiation out by a prism or grating. In much of the earlier 

 work this was not attempted, and it was really by the invention of the 

 bolometer by Professor Langley that the detailed study of quality as well 

 as quantity of radiation was rendered practicable. But a great deal of 

 valuable knowledge was obtained by the earlier workers who merely 

 investigated radiation totals, and we shall give here a brief account of 

 their results. 



Comparison of Emissive Powers. Leslie* compared emissive 

 powers by an arrangement represented in Fig. 133. C was a tin cube 



FIG. 133. Leslie's Comparison of Emissive Powers. 



filled with boiling water. M was a mirror of tinplate beaten into parabolic 

 form and about 12 inches aperture, th was the thermometer bulb placed 

 at the focus conjugate to C. The cube had its vertical face either plain 

 or coated with the substance to be tested. He obtained the following 

 results : 



Emissive Power of Surfaces at 100. Lampblack = 100. 



Lampblack . . . . . . . 100 



Writing paper 98 



Indian ink ....... 88 



Bed lead . 80 



Tarnished lead 45 



Clean lead .19 



Polished iron ....... 15 



Tinplate, gold,'! 12 

 silver, copper / 



Melloni obtained results confirming those of Leslie, but De la Provostaye 

 and Desains showed that through faulty arrangements both the earlier 

 observers overestimated the emissive power of metals. They obtained 



* An Experimental Inquiry into the Nature and Propagation of Heat. 180$. 



