306 Maxwell. 



A different way of inferring the necessity for light-pressure 

 was indicated in 1876 by A. Bartoli,* who showed that, when 

 radiant energy is transported from a cold body to a hot one by 

 means of a moving mirror, the second law of thermodynamics 

 would be violated unless a pressure were exerted on the mirror 

 by the light. 



The thermodynamical ideas introduced into the subject by 

 Bartoli have proved very fruitful. If a hollow vessel be at a 

 definite temperature, the aether within the vessel must be full 

 of radiation crossing from one side to the other : and hence the 

 aether, when in radiative equilibrium with matter at a given 

 temperature, is the seat of a definite quantity of energy per 

 unit volume. 



If U denote this energy per unit volume, and P the light- 

 pressure on unit area of a surface exposed to the radiation, we 

 may applyf the equation of available energy! 



U-T dF P 

 ~ 1 dT 



Since, as we have seen, 



this equation gives . dU 



dT' 



and therefore U must be proportional to T*. From this it may 

 be inferred that the intensity of emission of radiant energy by 

 a body at temperature T is proportional to the fourth power of 

 the absolute temperature a law which was first discovered 

 experimentally by Stefan in 1879. 



In the year in which Maxwell's treatise was published, 

 Sir William Crookes|| obtained experimental evidence of a 

 pressure accompanying the incidence of light; but this was 



* Bartoli, Sopra i movimenti prodotti dalla luce e dal calore e sopra il radiometro 

 di Crookes. Firenze, 1876. Also Nuovo Cimento (3) xv (1884), p. 193 ; and 

 Exner's Rep., xxi (1885), p. 198. 



t Boltzmann, Ann. d. Phys. xxii (1884), p. 31. Cf. also B. Galitzine, Ann. d. 

 Phys. xlvii (1892), p. 479. 



| Cf. p. 240. Wien. Ber. Ixxix (1879), p. 391. 



|| Phil. Trans, clxiv (1874), p. 501. The radiometer was discovered in 1875. 



