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XIV. On Radiant Energy. By B. G-alitzine*. 

 § 1. Introduction. 



STARTING with the far-reaching ideas of Faraday, Max- 

 well f has developed his theory of dielectrics, in which, 

 hy the identification of light with electrical vibrations, he 

 comes to the conclusion that a ray of light must exert a 

 certain pressure in the direction of its propagation, this pres- 

 sure being numerically equal to the energy contained in each 

 unit of its volume. One half of this energy is present in the 

 electric, and the other half in electromagnetic, form. 



By an entirely different line of argument, derived from an 

 application of the second law of thermodynamics, Bartoli :£ 

 has arrived at the same result. His paper is extremely inter- 

 esting, and the method by which he proves the existence of 

 the pressure of light is free from objections, at any rate in 

 the form given to it by Boltzmann § in a more recent paper. 

 But the process by which he calculates the numerical value 

 of this pressure P appears to me to be wrong. Bartoli 

 imagines a perfectly reflecting empty sphere of radius R, with 

 a perfectly non-reflecling (black) sphere of very small radius 

 r at its centre. Let Q be the quantity of energy which falls 

 on each unit of surface of the outer sphere in unit time. 

 Then if the radius of this sphere be decreased by 8R, accord- 

 ing to Bartoli the inner sphere receives an amount of heat 

 q = 2Q/V . 47rR 2 . SR, where V is the velocity of propagation 

 of light. In this expression 2Q/V is the energy contained in 

 unit of volume. Whether the energy can be expressed so 

 simply or not, is not evident without further explanation. 



2 

 For the case of a cylinder, = times the energy which is 



received by any normal section of the cylinder does not repre- 

 sent the energy contained in unit volume, as we shall see later. 

 Boltzmann || has already pointed out that Bartoli does not 

 appear to have considered the effect of rays with oblique 

 incidence. 



Bartoli goes on to say that, since the inner sphere 

 has increased its energy by an amount q, the work done 



* Translated from Wiedemann's Annalen, vol. xlvii. pp. 479-495 

 (November 1892), by James L. Howard, D.Sc. 



t 'Electricity and Magnetism/ vol. i. p. 144; vol. ii. p. 393 (2nd edit.). 



% Sopra i movimenti pi-odotti dalla luce e dal calore e sopra il radio- 

 metro di Crookes (Florence. Le Monnier, 1876). Also Num. Cim. [3] xv. 

 pp. 193-202 (1884) ; Exner's Repertorium, xxi. pp. 198-207 (1885). 



§ Wied. Ann. xxii. p. 33 (1884). 



|| Tom. cit. p. 35. 



Phil. Mag. S. 5. Vol. 35. No. 213. Feb. 1893. I 



