4 Jr. Ferret — Law of Thermal Radiation. 



fan of somewhat recent date.* The object of the present 

 research is to compare these formulae with the principal data 

 on hand derived from experiment and observation, and to ascer- 

 tain how nearly they represent the true law, and what modifi- 

 cations of these formulae, if any, are still required in order to 

 this. The want of space will forbid my giving an}' detailed 

 accounts of the experimental data used, and so for these the 

 reader will have to consult the references. Stefan has done 

 some important work in this line of research, and some of his 

 data will be used here and some of his results will be briefly 

 given. 



2. Let H = the rate with which heat is radiated by a body 

 from each unit of surface, 

 r = the temperature of the radiating body, 

 m = the value of H at the temperature of r = 0. 



If we now put 



(1) H=ma T , 



this, in the special case in which «=l , 0O77 becomes the expres- 

 sion of Dulong and Petit's law. 



But if the body is not in empty space, but is contained 

 within a perfect enclosure of temperature r , then by Prevost's 

 law of interchanges the body receives upon each unit of sur- 

 face an amount of heat H = r/ia T °, and hence we have for the 

 rate with which each unit of surface of the body loses heat, 



(2) H— H =;/2a r — ma T ° = ma Tl) (a d — 1), in which (3) S=t — t . 



If we now let 



R = the rate of cooling of the body, 



C = its thermal capacity, supposed to be the same for 



all temperatures, 

 c = its specific thermal capacity, 

 <j = its specific gravity, 

 s = the area of radiating surface, 



we then have 



(4) R=A(a d -l), in which (5) A. 



msa' 



C - 



In the special case of a spherical body of radius r this be- 

 comes 



(5) A= — a' . 



x ' rc<3 



For inclosures of different temperatures it is seen that these 



expressions of A vary, with a change of temperature of the 



inclosure, as a T °. Where the inclosure is not perfect, as where 



* Sitzungsb. Akad. Wien, II, lxxix, 391, 1879. 



