Intelligence and Miscellaneous Articles, 391 



number of these molecules is proportional to the intensity of the 

 current, and varies but little with the variations of pressure as long 

 as the discharge retains its character (III. 1 and 2). This radiation 

 is of an irregular character according to M. R. von Helmholtz {Die 

 Licht unci Warmestrahlung verhrennender Gase, Berlin 1890) ; or it 

 belongs to the class of phenomena to which M. E. Wiedemann 

 (Wied. Arm. xxxvii. p. 177, 1889) has given the name of phenomena 

 of luminescence in this sense, that the composition of the radiation 

 does not correspond with the temperature of the gas. The radia- 

 tion may be of extreme intensity in the groups of short wave- 

 lengths. 



When the pressure increases and with it the number of inactive 

 molecules, the irregular radiation of the active molecules may more 

 easily be transmitted to the former. The number of radiating 

 molecules increases therefore, and consequently also the radiation of 

 the gas, but it changes in character in this sense, that the radiation 

 of the groups of shorter wave-lengths relatively diminishes. — 

 Ofversigt af Kongl. Akad. Forhandl., 1891, No. 6. 



ON THE REPULSIVE FORCE OF RADIATING BODIES. 

 BY PETER LEBEDEW. 



It was shown by Maxwell * that a pressure is exerted on 

 absorptive bodies by incident rays in the direction of their pro- 

 pagations, and that this can be expressed in the form 



p =v & 



in which E is the energy imparted to the body by the incident 

 rays in unit time, and v the velocity of light in the medium in 

 which the body is placed. 



Independently of Maxwell, Bartolit and BoltzmannJ arrived at 

 analogous results in treating the case of reflexion. Bartoli found 

 that at perpendicular incidence the rays exert a pressure against 

 a mirror, which is twice as much as that which, according to 

 Maxwell, they would exert against an absorbent body. 



In what follows the ratio of the repulsion produced by radia- 

 tion to the Newtonian attraction is deduced both for our sun, 

 and for a hot spherical body in general. The expressions obtained 

 hold only for absolutely black bodies, wmose dimensions are 

 great in comparison with the wave-length of the incident 

 radiation ; accordingly the interesting questions respecting the 

 repulsive force in the tails of comets, and the mutual action of 

 two adjacent molecules in the body cannot be solved, but only 

 indicated . • - • 



As it is only the order of the magnitude of the desired ratio 



* Electricity and Magnetism, § 792. 



t Exner's liepertorium, vol. xxi. p. 198 (1885). 



t Wied. Ann. vol. xxii. p. 31 (1884). 



