Physics. — “Gravity and Pressure of Radiation.” By H. Groor. 
(Communicated by Prof. W. H. Juttus.) 
(Communicated at the meeting of June 25, 1921). 
§ 1. In 1910 LeBeprw succeeded in experimentally showing 
pressure of radiation on gases, and measuring the value of the pressure. 
Since then attempts to take this force into account in astro-physical 
researches have not been wanting. 
Particularly Eppineton*) and Jans’) have reached highly remark- 
able results concerning the structure of “giant stars” by introducing 
besides gravity, also pressure of radiation into their equations. They 
come, among other things, to the conclusion that through the in- 
fluence of this pressure, the gravity in the interior of a star can 
be considerably diminished, and this the more as the density is 
smaller. 
One is naiurally led to extend this investigation to states as will 
probably be met with in nebulae. And this the sooner as different 
authorities advocate the hypothesis that the law of Newron is not 
valid during the nebulous stage of a star, i.e. during the period that 
the star is being formed from primitive nebular matter. 
Kapreyn *) and CAMPBELL *) tried to account in this way for the 
surprising fact that the proper motions of the stars increase as a more 
advanced spectrum type is examined. The latter indicates a possible 
pressure of radiation as a force that might partially neutralize gravity. 
Also F. N6LKE*) in his cosmogonic considerations has recourse 
in numerous places to the pressure of radiation to render the not 
being constant of gravity plausible. 
An estimation of the extent of the possible effect is, however, no- 
where found. And so long as this is wanting all conclusions which are 
exclusively based on qualitative speculations, remain unreliable — as 
but too clearly comes to light in the different cosmogonies. 
1) M.N. 77, (1916—17), p. 16 and p. 596; Astrophys. J. 48, (1918). 
3) M. N. 79, (1919), p. 319. 
5) J. C. Kapreyn, Astrophys. J.*1910 (April). 
4) Camppe.t, Lick-Observ., Bulletin. Vol. VI N°. 196. 
6) „Das Problem d. Entwicklung unseres Planetensystems”, 2te Ausgabe (1919) 
Berlin; A. N. 188, (4509). 
