NICHOLS AND HULL. — PRESSURE DUE TO RADIATION. 561 



In 1873 Maxwell, * on the basis of the electromagnetic theory, showed, 

 that if light were an electromagnetic phenomenon, pressure should result 

 from the absorption or reflection of a beam of light. After a discussion 

 of the equations involved, he says : " Hence in a medium in which waves 

 are propagated there is a pressure in the direction normal to the waves 

 and numerically equal to the energy in unit volume." Maxwell com- 

 puted the pressure exerted by the sun on the illuminated surface of the 

 earth and added : "It is probable that a much greater energy of radiation 

 might be obtained by means of the concentrated rays from an electric 

 lamp. Such rays falling on a thin metallic disc, delicately suspended in 

 a vacuum, might perhaps produce an observable mechanical effect." 



Apparently independent of Maxwell, Bartoli f announced in 1876 that 

 the Second Law of Thermodynamics required the existence of a pressure 

 due to radiation numerically equal in amount to that derived by Maxwell. 

 Bartoli's reasoning holds for all forms of energy streams in sj^ace and is 

 of more general application than Maxwell's equations. Bartoli contrived 

 elaborate experiments to verify this theory, but was balked in the search, 

 as all before him had been, by the complicated character of the gas 

 action which he found no way of eliminating from his experiments. 



After Bartoli's work the -subject was dealt with theoretically by Boltz- 

 mann,t Galitzine,§ Guillaume, || Heaviside,1[ and more recently by Gold- 

 hammer.** Fitzgerald, ft Lebedew,t| and Hull §§ have discussed the 

 bearing of radiation pressure upon the Newtonian law of gravitation with 

 special reference to the repulsion of comets' tails by the sun. Arrhenius |||| 

 has recently discussed the cosmical consequences of radiation pressure not 

 only concerning comets' tails, but, by combining radiation pressure with 

 the known properties of negative ions, has endeavored also to account 



* J. C. Maxwell, A Treatise on Electricity and Magnetism (1st Edition), II. 391. 

 Oxford, 1873. 



t A. Bartoli, Sopra i movementi prodotti della luce e dal calorie, Florence, 

 Le Moniiier (1870), also Nuovo Cimento, XV. l'J3 (1881). 

 t L. Boltzmann, Wied Ann., XXII. 31, 291 (1884). 

 § B. Galitzine, Wied. Ann., XLVII. 479 (1892). 

 II Ch. Ed. Guillainne, Arch, de Gen. (3), XXXI. 121 (1804). 

 1 O. Ileaviside, Klectroinagiietic Theory, I. 334. London, 1893. 

 ** D. A. Goldhammer, Ann. Phys., IV. 834 (1901). 

 tt G. F. Fitzgerald, Proc. Koy. Soc. Dub. (1881). 



U P.Lebedew, Wicd.Ann.,XLV.292(1892). Astrophya." Jmir., .\1V. 155(1902). 

 §§ G. F. Hull, Trans. Astron. See. Toronto, p. 123 (1001). 



nil S. Arrhenius, Konigl, Vetanskaps. Akademiens Fiirdliandlingar, j). 545 

 (1900). 



VOL. XXXVIII. — 30 



