S22 Dr. Megli Nad Saha on 



regard to future work on the subject : — Elements like Fe, 

 Ti, V, Ni have probably ionization poientials varying from 

 6 to 9 volts, so that their behaviour is almost parallel to that 

 of calcium and magnesium. Elements which are missing 

 from the sun can be broadly subdivided into two groups : 1st, 

 those which are completely ionized, e.g. Wo, Cs, and pro- 

 bably thallium ; 2nd, elements of which the ionization and 

 radiation potentials are so high that they are not in a state 

 capable of absorbing those of their characteristic lines which 

 occur in the continuous photospheric spectrum {vide paper 

 0). Helium and most of the inert gases fall within this 

 group. The case of helium has already been considered. 



Neon, Argon*, and other inert gases have very high ioni- 

 zation potentials, the value being 16 volts for argon. Their 

 principal emission lines (1, s)~ (2, p) lie in the ultra-violet, 

 and identification is to be carried on by lines (2, s) — (m, p), 

 or (2,p) — (m, d) or (2, p) — (m, s). As the intensity of the 

 photospheric radiation in the region (1, s)—(2, p) is very 

 small, the compensating value of the radiation- pressure is 

 also small, and the elements fail to reach great heigh! s. In 

 the lower regions, the stimulus is not sufficient enough to 

 convert the atoms to the states (2, s) or (2, p) and hence 

 these lines do not occur in the Fraunhofer spectrum f. 



The case of the unidentified Fraunhofer Lines. 



As has been mentioned in the introduction, about 60 per 

 cent, of the Fraunhofer lines catalogued by Rowland still 

 remain to be identified. According to Fowler, a large 

 proportion of these are due to molecules and molecular com- 

 pounds. 



The best known example is the so-called cyanogen band 

 with its head at \ = 3883, now attributed by Grotian and 

 Eunge to molecular nitrogen. Other examples are Newall's 

 identification of the G-group of the Fraunhofer spectrum with 

 the band-lines of hydrocarbons, and Fowler's detection of 

 ammonia and water-vapour bands in the ultra-violet part 

 of the solar spectrum. 



These identifications raise a very important point, viz., Is 

 it possible, under the conditions prevailing in the sun, for 



* For the spectral grouping of the lines Ne and A, see Paschen, Ann. 

 d. Pliysik, vol. lx. 



t Probably the metalloids As, Sb, Bi, etc., fall in this category, for 

 it is well known that they can be made to emit their line spectrum at a 

 comparatively low temperature, vide paper C, Introduction. 



