in the Solar Chromosphere. 483 



alkaline earths in vacuum-tube furnaces at varying tem- 

 perature. Unfortunately, the pressure, which is a vital 

 point, is not mentioned. The last line shows the percentage 

 of the ionized atoms under a pressure of 10 ~ 4 atmosphere, 

 or *1 mm. of mercury. 



The tables show that an increase of temperature causes an 

 increase of ionization and the proportion of emission centres 

 of the enhanced lines. The increasing intensities of the 

 double lines are mainly to be ascribed to this fact. These 

 become comparable in intensity to the principal lines of the 

 normal atom only when the degree of ionization is rather 

 large (comp. the figures at 4000°). Comparing the relative 

 intensities of the corresponding lines of the calcium and 

 barium group, we find that for the same temperature the 

 enhanced lines of barium are relatively stronger than the 

 calcium lines ; and this, according to our theory, is due to 

 the comparatively lower ionization potential of barium. 



The objection may be raised whether the proportion of 

 ionized atoms at low temperatures, as given by the theory, 

 is not rather too low. The tables show that at 2000° K., 

 only 1 in 10° calcium atoms is ionized. Is this small 

 number of ionized atoms capable of affecting the photo- 

 graphic plate by the emission of the H and the K lines? 



No definite answer can be given to this point. We may, 

 however, point out that, according to Ladenburg and Loria*, 

 when a hydrogen vacuum tube at a pressure of a few mms. 

 of mercury is excited by a spark, only 1 atom in 50,000 is 

 found to be radiant, i. e., capable of emitting H a and H^. 

 A very low proportion of radiant centres may therefore 

 affect the photographic plate. It should also be remembered 

 that at low temperatures the principal line of the normal 

 atom is not only relatively more intense, but very broad and 

 diffuse, when the enhanced lines are extremely narrow in 

 addition to being faint. 



A reduction in pressure will cause the relative intensity 

 of the H-K line to increase, but not the absolute intensity, 

 because the total available number of radiant particles will 

 now decrease. I am not aware whether there is any 

 laboratory experiment for testing this point. 



§ 4. Hydroijen in the Sun. 



It has been mentioned in the introduction that hydrogen 

 is not appreciably ionized at even the highest levels of the 



* Ladenburg and Loria, Ber. d. I). Phys. Gesellschaft, 1908. 



