﻿Disk rotating in a Vacuum. 31 



In this formula $ denotes the temperature of the surrounding 

 medium, t the excess of the temperature of the heated body, p the 

 pressure of the surrounding air; the other signs denote constants. 



From the formula adduced, the value of the coefficient of 

 thermal radiation is obtained by division by the value t, assumed 

 to be very small. We have thus 



h = ma*^j^ +np c t<>-\ 



From Dulong and Petit' s determination we have, in Centigrade 

 degrees, 



a= 1-0077, 

 6 = 1-233, 

 c=0-45. 



I have to thank Professor Neumann for the statement that for 

 a blackened surface 



m = 3'6 ; 



and the coefficient n, which is independent of the nature of the 

 surface, in case p is expressed in atmospheres, is for atmospheric 

 air 



?z=0-0168. 



The first number is deduced from his own observation, the 

 latter from occasional statements of Dulong and Petit. Both 

 numbers refer to Paris lines and minutes as units. 



Using these values, and taking from Stewart and Tait's ob- 

 servation 



^ = 0010 atmosphere, 

 * = 0°-45 C, 



and putting 3= about 20° C, we get from the above formula the 

 value 



A = 0-0013, 



expressed, again, in millimetres and seconds of time. 



The concordance of the value deduced above from Stewart and 

 Tait's observation with this directly found is greater than was to 

 be expected from the multifold uncertainty of the observations. 



Another beautiful agreement is also met with. According to 

 a communication of Professor Neumann, for a metallic surface 



m= about 0*5, 



that is, about one-seventh that of a lampblack surface. We get 

 from this for the radiation-constant h of a metallic surface the 

 value 



7^=0-00023; 



