ASTRONOMY 3 



corresponding to various pressures and temperatures for 

 any element whose ionisation potential has been determined. 

 Taking, for instance, the case of calcium, Saha finds that the 

 ionisation will be practically complete for a temperature of 

 14,000" at one atmosphere pressure ; for a temperature of 

 11,000° at i/io atmosphere; of 9,000° at i/ioo atmosphere ; 

 of 7,000° at 1/10,000 atmosphere, etc. Assuming that the tem- 

 perature in the photosphere is 7,500° with a pressure of the 

 order of one atmosphere, falling to 6,000° in the outermost 

 layers with a partial pressure of io~'^ atmospheres, the extent 

 of the ionisation in the solar chromosphere may be determined. 

 In the case of calcium, therefore, when the pressure falls to io~* 

 atmosphere, practically all the atoms get ionised. Hence up 

 to this point, the combined emission of the HiK and ^-lines 

 is obtained, but above this point there is only that of the H 

 and K lines. In the case of strontium and barium, which 

 have a lower ionisation potential, ionisation is practically 

 complete at io~^ atmosphere, so that the heights shown by the 

 lines of the unionised atoms of these elements are still lower. 

 Hydrogen, on the other hand, is completely dissociated into 

 atoms throughout the chromosphere, but is not appreciably 

 ionised except at the very highest levels. For helium to have 

 appreciable ionisation, a temperature of at least 16,000° is 

 necessary. In a similar way, Saha shows that the theory 

 accounts for the behaviour of sodium lines in the Fraunhofer 

 spectrum and for their intensification in the spot spectrum ; 

 for the faint occurrence of the potassium lines and for the 

 complete absence of the lines of caesium and rubidium. It 

 therefore seems that the theory will be able to account for the 

 presence or absence of the lines of various elements in the 

 Fraunhofer spectrum, a complete explanation not being 

 possible at present on account of lack of information as to the 

 ionisation potentials of many elements. No evidence has 

 been found of the existence in the sun of rubidium, caesium, 

 nitrogen, phosphorus, boron, antimony, bismuth, arsenic, 

 sulphur, selenium, thallium, praseodymium. The following 

 elements are doubtful : radium, the inert gases (except helium), 

 osmium, iridium, platinum, ruthenium, tantalum, thorium, 

 tungsten, uranium. The following elements are represented 

 only by faint lines : potassium, copper, silver, cadmium, zinc, 

 tin, lead, germanium. Certain elements such as chlorine, 

 bromine, iodine, fluorine, tellurium, have not been investigated. 

 A few elements, such as calcium, iron, vanadium and titanium, 

 are unusually prominent. The theory confirms these facts 

 where it can give any information, and strongly supports the 

 view that the varying records of different elements in the 

 Fraunhofer spectrum may be regarded as arising from the varying 



