676 Prof. F. A. Lindemann on the 



be able to scatter light unless it recombines. It seems 



natural to suppose that the chance of scattering a quantum 



hv is proportional to the intensity of radiation of frequency v 



present in the incident radiation, i. e. proportional to 



v 3 

 - lv in the case of complete radiation. Without as- 



<?**— 1 



suming some definite atomic model it is impossible to form 

 any accurate idea of what happens in a many-line spectrum. 

 The fundamental fact remains, however, that the intensity of, 



say, the blue hydrogen line of frequency v= ~ , (i — iV) 



is about 2. 10*" as great as the intensity of the last hydrogen 



2ir q me 4 



line of frequency v = - — j-, — which corresponds to ioniza- 

 tion. One might reasonably expect, therefore, that some- 

 thing like 10 6 quanta would be scattered before the atom was 



ionized. Taking u = ^ — and m-=l"65.10 -2 *, the velocity 



a 8/* 3 ■ J 



increment will be 82 cm./sec. each time a quantum is 

 scattered by a hydrogen atom. It does not seem unlikely, 

 therefore, that such atoms will on the average acquire 

 a velocity of the order of 8.10 7 cm./sec. before being ionized. 

 Prom this one may conclude that light-pressure is of the 

 right order to explain the velocities observed in protuber- 

 ances, and that there is reason to believe these to consist of 

 an equal quantity of positive ions and electrons. 



That such clouds of gas will be highly ionized may be 

 shown in a more satisfactory way which takes account of 

 recombination. Taking the case of hydrogen, which is the 

 only substance about which enough is known to enable 

 quantitative results to be obtained, it is not difficult to form 

 an estimate of the ionization in terms of the temperature and 

 pressure. As is well known, the equilibrium constant 



~K p = ' l iJ' , where p l5 p. 2 , and p' are the partial pressures of 

 ions, electrons, and atoms respectively, is given by 



ff^PsC/ff + S.'. 



eY 

 kT 



*'o 



2ir 2 me 4: 



Here eV = hv =z , a — is the work necessary to ionize the 

 atom, h is Boltzmann's constant ^ , C p is the atomic heat, 



