GENERAL ACCOUNT OF RADIATION. 



229 



source and the thermopile, the side towards the source being covered 

 with the substance and that towards the pile with lampblack.* 



If the quantity of heat emitted in one second by one side, per degree-rise 

 of temperature of the plate above the surroundings, is E for lampblack, and 

 E t for the substance, and if the excess of temperature is t v the emission is 



If the quantity absorbed by the surface towards the source is A x , then in 

 the steady state 



If both surfaces are lampblacked, and the excess of temperature is t, then if 

 A is the quantity absorbed by the lampblacked surface towards the source, 



Then 



2E~ 



E 



Now the indications of the galvanometer in circuit with the pile are 



/ ~F 



proportional to ^ and t, so that we know -i. Knowing also J from the 



t -El 



J^ 



previous determinations of emissive power, we can find -i, 



A. 



Just as the light absorbed by various surfaces depends on the nature 

 of the incident light, red paper, for instance, absorbing more of blue light 

 than it does of red, so the absorptive power for radiation in general 

 depends on the source. For example, among other results Melloni 

 obtained the following absorptions from the sources given at the head 

 of the columns, and taking in each case the absorption of lampblack 

 as 100: 



De la Provostaye and Desains worked in another way, coating a 



* A simple method of illustrating the difference of absorptive power of metal and 

 lampblack consists in placing two equal metal vessels in front of some source of 

 heat equidistant, say, from a gas-flame, the surface of one being lampblacked on 

 the side turned towards the source. The rise in temperature of the lampblacked 

 vessel is much more rapid than that of the other. A good way of arranging the 

 experiment is to make each of the vessels into an air thermometer on the principle 

 of Fig. 32, chap. iv. 



