EMISSIVITY. 



Tables 244, 245. 



TABLE 244. — Constanta of Emlsslvlty. 



The constants of radiation into vacuum have been clcterminccl for a few substance.s. 'I"hc 

 object of .several of the investigations has been (he cictcrminatiun of the law of variation with 



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

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



quation 



H-- 



w'hcre ^ is a constant depending on the units em|)ioyed and on the nature of the surface, s the 

 surface, (7 a constant delcrminL'd by Dulong and Petit to be 1.0077, fl the absolute temperature 

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

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

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



Glass // = .00001327 



Dry chalk ^^ = .00001 195 



Dry new red-sandstone A ^= .00001162 

 Sandstone (building) . 

 Polished limestone . . 

 Unpolished limestone 

 (same block) . . . 



Stefan's law is expressed by the equation 



II=<Ts{T^^-T^% 



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

 stant, T\ is the absolute temperature of the radiating body and To 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 1° absolute temperature to 

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

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



Graetz * finds for glass ^ 1 



Schleiermachcr t find for polished platinum wire . < yl 



For copper oxide < y} 



Tx = 400, 71) = o, (T = 1.0846 X 10-12 

 10S5, 7o = o, <r = 0.185 X 10-12 

 1 1 50, 7'o = o, <r = o.i77 X IQ-I^ 

 850, 7^) = o. cr = 0.600 X 10-12 

 1080, 7o = o, (r = 0.701 X IQ-^ 



TABLE 245. — Effect of Absolute Temperature of Surface. 



The following tabular results are given by Bottomley. t The results of Sclileierniacher were cilculated from data given 

 in the paper above quoted. The temperatures /, are in degrees centigrade, and c is ihe 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. 11, p. 297. 

 t " Wied. Ann." vol. 2^1, p. 305. 

 t " Phil. Trans. Roy. Soc." 1887, p. 429- 



