On Planck's Law of Radiation. 



229 



were found to agree well with each other. They are given 

 below : — 







Date of Exposure 



to Emanation. 21st Sept., 1915. 







Date of 



testing. 



Plate 1. 



Plate 2. 



Mean E . 



P in 



div./min 



A in j 



diY./imn. 



1 ' 



E. 



P in 

 div./min. 



A in 



div./min. 



3. 



E. 



31.1.16 



1.5.16 



2-18 

 354 



S3 -161 

 1-46 j -161 



•657 



1 

 •696 



1-93 

 3-37 



•78 

 1-315 



•186 

 ■186 



•707 

 •690 



•687 



Inserting the value E = '687 in equation (J) we obtain 



-=•626. 

 c 



The ratio of the ranges of the a particles from radium A 

 and radium C in platinum seems not to have been deter- 

 mined, but the value of this ratio in gold can be deduced 

 from the experiments of Marsden and Richardson to be 

 '12* ; and since the atomic weights of gold and platinum 

 are nearly equal, this number must apply with considerable 

 accuracy to the case of platinum. There is therefore more 

 difference between the ranges of the recoil-streams from 

 radium A and radium than of the corresponding a particles, 

 and there can be no doubt that this difference is real, for 

 experimental errors such as contamination of the platinum 

 surfaces would tend to reduce the efficiency of recoil and so 

 increase the value of the observed ratio a/c. 



Physical Laboratory, 

 The University, Manchester. 



XXIV. A Method of Deriving Planch's Law of Radiation. 

 By George Green, D.Sc, Lecturer on Natural Philosophy 

 in the University of Glasgow^. 



THE following method of deriving Planck's law was sug- 

 gested by Lo r d Rayleigh's paper on "The Dynamical 

 Theory of Gases and of Radiation " % . We take the radiating 

 body to be a gas contained in a perfectly reflecting enclosure 

 in thermal equilibrium at temperature 6 ; and the investiga- 

 tion is b:ised on the following assumptions: — 



* Marsden and Richardson, Phil. Mag. Jan. 1913. 

 t Communicated by Professor A. Gray, F.H.iS. 

 t Scientific Papers, vol. v. p. 248. 



