A different way of inferring the necessity for light-pressure was indicated in 1876 by A. Bartoli,[1] 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 apply[2] the equation of available energy[3]
.
Since, as we have seen, , this equation gives , and therefore U must be proportional to T4. 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[4] in 1879.
In the year in which Maxwell's treatise was published, Sir William Crookes[5] obtained experimental evidence of a pressure accompanying the incidence of light; but this was
- ↑ Bartoli, Sopra i morimenti 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.
- ↑ Boltzmann, Ann. d. Phys. xxii (1884), p. 31. Cf. also B. Galitzine, Ann, d. Phys. xlvii (1892), p. 479.
- ↑ Cf. p. 240.
- ↑ Wien, Ber. lxxix (1879), p. 391.
- ↑ Phil. Truns. clxiv (1874), p. 501. The radiometer was discovered in 1875.
