u; />,.,.,./_ Weber>g Law qf Thermal Radiation. 139 



ler. With the results of those experiments the first member of 



was found to be \vrv nearly a constant through a range of 



temperature of about 700°, which is remarkable for so great a 



range. . 



Schleiermacher lias himself tested Webers formula in the 



ie way by using the results of his more recent experiments 

 by the electric method," and found the formula fairly satisfac- 

 tory, hut these experiments extended through a range of not 

 more than L50°, and less in the one ease. 



;.. The formula, so far, has been compared with the results 

 only of Schleiermacher's experiments, but when we come to 

 test it by means of experimental results in which the absolute 

 rate of losing heat is determined from the observed rate of 

 cooling of heated bodies of known thermal capacity, and the 

 relative rate, from the galvanometer needle of the thermopile, 

 the law is not so satisfactory. < t 



If we take the corrected rates of cooling R obtained in 

 Dulong and Petit's experiments, given in the table of §4, paper 

 A, and given here in the second column of the following 

 table, in which T 1 = 273°, we get the corresponding values of 

 CFT, . e aTl with ^ = -004-3 as given in Weber's formula. 







CFTx . e aTl with 



T 



AS T? Tl 







' 









a—0043 



a=-0064 



353° 



148° 



1-799 



1-160 



373 



1-96 



1-782 



9906 



393 



2-60 



1-842 



0-922 



413 



3 38 



1-917 



0-964 



433 



4-31 



2-000 



1005 



453 



5 43 



2-090 



1-062 



473 



6-64 



2-145 



1 035 



493 



7-95 



2-178 



0955 



513 



974 



2279 



1003 



The values in the second column here are rates of cooling 

 and not the rates of given out heat in absolute measure as the 

 formula requires, but the two are sensibly proportional since 

 the capacity of mercury for heat changes but little with change 

 of temperature. The 'values of the third column, therefore, 

 should still be a constant. It is seen that this is far from being 

 the case, but that there is an increase with increase of tempera- 

 ture, indicating that a greater value of a is required in the 

 formula to satisfy the condition of (4), which the law requires. 

 But if we increase the value of this constant about one half, or 

 to -0064, we get the results of the last column of the table, 



* Wied. Annalen, xxxiv, 623. 



