394 Prof. E.Wiedemann on the 



mutual forces between the separate atoms, the respective 

 spectra must be altogether different. 



Store of Luminous Energy and Specific Heat of Monatomic 



Gases. 



40. The determination of the store of luminous energy 

 gives us also a means of explaining an otherwise somewhat 

 puzzling circumstance. The experiments of Messrs. Kundt 

 and Warburg* have shown that mercury vapour behaves as 

 to its specific heat as a monatomic gas, i. e. that upon heating- 

 it is only the velocity of translation of the molecules which 

 increases, but no internal energy is produced in each atom. 

 This result is contradicted by the fact that mercury vapour 

 gives a very fine bright spectrum of lines, which certainly 

 must result from intra-molecular motions. As we do not yet 

 possess the data to calculate the store of luminous energy in 

 mercury, we will employ the results for sodium in order to 

 discover the cause of this apparent contradiction. 



Sodium vapour must, as the line-spectrum certainly shows f , 

 behave as a monatomic gas at the temperature of the flame. 



If we take the atomic heat of sodium vapour as the same 

 as that of mercury in the gaseous condition, viz. 3*0, then its 

 specific heat is 0'13. The total heat- contents W, which is 

 given to 1 gramme of sodium vapour when heated from the 

 absolute zero of temperature to 1000°, is then 0*13 x 1273, 



W= 165-5 cal. 



The store of luminous energy L, however, is only 



L = 3-2xl0- 5 cal. 



i. e. -r 



=t = 2x 10-7 a bout. 

 W 



The luminous energy is thus an immeasurably small frac- 

 tion of the total energy, and need not be taken into account 

 in observations of the specific heat. This would also be true 

 if, as is the case, separate sodium lines existed in the infra- 

 red. 



The conditions are otherwise if we compare the quantities 

 W and L for glowing platinum. 



If we put the mean specific heat of platinum between 



* Pogg\ Ann. cliv. p. 353 (1875). 



t Cf. H. v. Helmholtz in a work by Moser, Pogg. Attn. clx. p. 182 

 (1877), and E. Wiedemann, Wied. Ann. v. p. 501 (1878). 



