TABLES 244, 245. 

 EMISSIVITY. 



TABLE 244. - Constants of Emissivity. 



The constants of radiation into vacuum have been determined for a few substances. The 

 object of several of the investigations has been the determination of the law of variation with 

 temperature or the relative merits of Dulong and Petit's and of Stefan's law of cooling. 



Dulong and Petit's law gives for the amount of heat radiated in a given time the equation 



H=Asa e (a t i) 



where A is a constant depending on the units employed and on the nature of the surface, s the 

 surface, a a constant determined by Dulong and Petit to be 1.0077, # the absolute temperature 

 of the enclosure, and t the difference of temperature between the hot surface and the enclosure. 

 The following values of A are taken from the experiments of W. Hopkins, the results being 

 reduced to centimetre second units, and the therm as unit of heat. 



Glass . . . ;';-' . . .,4 = .00001327 



Dry chalk /4 = . 00001195 



Dry new red-sandstone A = .00001162 



Sandstone (building) . ,<4 .00001232 



Polished limestone . . A = .00001263 

 Unpolished limestone 



(same block) . . . A = .0001777 



Stefan's law is expressed by the equation 



where //and s have the same meaning as above, a is a constant, called Stefan's radiation con- 

 stant, T\ is the absolute temperature of the radiating body and 7g the absolute temperature of 

 the enclosure. Stefan's constant would represent, if the law held to absolute zero, the amount 

 of heat which would be radiated per unit surface from the body at i absolute temperature to 

 space at absolute zero. The experiments of Schleiermacher, Bottomley, and others show that 

 this law approximates to the actual radiation only through a limited range of temperature. 



Graetz * finds for glass 



Schleiermacher f find for polished platinum wire 



For copper oxide 



7i = 400, T = o,tr = 1.0846 X IO" 12 

 ( 7\ = 1085, To = o, a- = o. 185 X io- 12 

 7\ 1150, 7b = o, IT 0.177 X io~ 12 

 T\ = 850, TO o. <r = 0.600 X io- 12 

 7\ = 1080, TO = o, <r = 0.701 X JO' 1 ' 2 



TABLE 245. Effect of Absolute Temperature of Surface. 



The following tabular results are given by Bottomley. t The results of Schleiermacher were calculated from data given 

 in the paper above quoted. The temperatures /, are in degrees centigrade, and e is the emissivity or amount of 

 heat in therms radiated per square centimetre of surface per degree difference of temperature between the hot body 

 and the enclosure. The results are all for high vacuum. 



SMITHSONIAN TABLES. 



* " Wied Ann." vol. u, p. 297. 

 t " Wied. Ann." vol. 26, p. 305. 

 t " Phil. Trans. Roy. Soc." 1887 



235 



