Mechanics of Luminosity. 389 



nated bodies there is always a portion of the radiated luminous 

 energy again replaced by the absorbed vibrations of light. 



(e) ' From determinations with solid phosphorescent bodies, 

 such as uranium nitrate, b is found to be about 10 3 . We will 

 treat this more fully in § 41. _ Values of the same order of 

 magnitude result from observations with other phosphorescent 

 bodies, in which no chemical transformations can be assumed. 



(/) From the time necessary for a platinum wire of 0*26 

 millim. thickness to cool from a bright white heat to darkness, 

 viz. 8 seconds, we obtain about 



5 = 100. 



The time taken as the basis of this calculation is, without 

 doubt, too great, since a portion of the radiated luminous 

 energy is again replaced by motions of translation. The value 

 of b is therefore greater. 



Evaluation of the Store of Luminous Energy. 



38. The following values, in gramme-calories, for the store 

 of luminous energy L were obtained according to the equation 

 L = E /ft, where E refers to the unit weight. 



We had found (§§ 26 and 21) for 



Sodium . . . E =3-2xl0 3 , 

 Platinum. . . E = 2'2xl0 4 , 

 If now we put for both substances, 



6 = 10 8 , 

 then the store of luminous energy is for 



Sodium . . . L = 3*2x 10 ~ 5 gramme-calories, 

 Platinum . . . L = 2'2xl0" 4 „ „ 



If we put b= 10 3 for platinum, which may be nearer the truth, 

 since we have here to do with a solid body, then the store of 

 luminous energy for platinum is 



L = 22 gramme-calories. 



These numbers represent in calorimetric units, as explained 

 above, the actual kinetic energy of the intra-molecular motions 

 which give rise to the emission of light under the above con- 

 ditions of luminosity. We thus obtain for the first time a 

 reliable insight into the order of magnitude of the energies of 

 these motions, and are thus in a position to obtain clearer 

 notions about these motions themselves. 



Proof that the Material Molecules are the Carriers of the Store 

 of Luminous Energy. Application to Spectra. 



39. We will now investigate whether the sethereal envelopes 



