Physical Constitution of the Sun. 297 



or 



a^m^L (12) 



Substituting for c its value from equation (7) and putting 



i =273, 

 a 



we get . 



This formula shows that the density (i. e. the mass contained 

 in a unit of volume) in a definite stratum of the chromosphere 

 can be calculated approximately, if the mass m i contained in the 

 above-defined space and the absolute temperature of the same 

 are known. 



§5. 



The expressions above found are worthy of notice for the pur- 

 pose of applying spectroscopic observations to the temperature- 

 relations of the sun ; for, provided we could with constant tem- 

 perature, observing the above-mentioned precautions, by varying 

 the pressure of electrically luminous gases produce those modi- 

 fications of the hydrogen-spectrum, for example, by which the 

 two limits of the chromosphere are optically determined, we 

 should be justified in presupposing in the sun's atmosphere also 

 the ratio of pressures herein found, and in this way, with the aid 

 of formula (1) or (3), ascertaining a numerical value for the 

 temperature of the stratum in question of that atmosphere. 



As is well known, the difficulties of fulfilling experimentally 

 the conditions here required are so serious on this account — 

 because in general the electrical resistance becomes greater as 

 the density of the gas increases, so that the greater quantity of 

 electricity necessary to overcome it produces at the same time a 

 higher temperature of the glowing gas. 



"Wullner has observed the spectrum of hydrogen under a great 

 variety of pressures. In his first memoir on this subject he 

 remarks : — 



"With 6 millims. pressure, besides the characteristic hydro- 

 gen-lines, the orange part was still just visible, 



" On the gas being still further rarified, to 3 and then to 2 

 millims., the characteristic lines retained the same brightness, 

 all the rest vanished almost completely from the spectrum. 

 Simultaneously, however, with a weakening of the bright lines 



