Tables 242, 243. 



EMISSIVITY. 



TABLE 242. — Emlsslvlty at Ordinary Pressures. 



According to McFarlane * the rate of loss of heat by a sphere 

 placed in the centre of a spherical enclosure which has a 

 blackened surface, and is kept at a constant temperature of 

 about 14^ C., can be expressed by the equations 



e = .000238 + 3.06 X io-«^ — 2.6 X io-«/2, 



when the surface of the sphere is blackened, or 



e = .000168 + 1.9S X lo-fif — 1.7 X IO-8/2, 



when the surface is that of polished copper. In these equa- 

 tions e is the emissivity in c. g. s. units, that is, the quantity 

 of heal, in therms, radiated per second per square centimetre 

 of surface of the sphere, per degree difference of tempera- 

 ture i, and t is the difference of temperature between the 

 sphere and the enclosure. The medium through which 

 the heat passed was moist air. The following table gives 

 the results. 



TABLE 243. — Emlsslvlty at Dlllerent Pres- 

 sures. 



Experiments made by J. P. Nicol in Tait's Labo- 

 ratory show the effect of pressure of the en- 

 closed air on the rate of loss of heat. In this 

 case the air was dry and the enclosure kept at 

 about 8^ C. 



Smithsonian Tables. 



* "Proc. Roy. Soc." 1872. 



t " Proc. Roy. Soc." Edinb. 1869, 



234 



