Physical Constitution of the Sun. 325 



these three atmospheres on the visible surface of the sun, the fol- 



g values : — 





Hydrogen . 



. p h = lS0 millims., 



Nitrogen 



. i>/i=323jQ73 millims., 



Oxygen . . 



i?A=124:j-Qg3 „ 



Hence it follows that, under the above conditions, the quanti- 

 ties of the two latter gases at the point in question, where the 

 hydrogen spectrum becomes continuous, may be considered per- 

 fectly inappreciable in comparison with the quantity of hydrogen. 

 This would still be the case if we were to assume that the weight 

 of each of the two atmospheres is many million times as great; 

 although, considering that nitrogen is fourteen times and oxygen 

 sixteen times as heavy as hydrogen, it would only take T \ and 

 -j-g- the quantities in order to make the pressure on these atmo- 

 spheres at the lowest level agree with that on the hydrogen. 

 The maximum value of the pressure at the lowest point of these 

 atmospheres must, in accordance with our previous hypothesis, 

 here be assumed to be equal to the mean density of the sun ; and. 

 it is easy by means of formula (III.) and the known specific 

 gravity of oxygen and nitrogen to calculate how great the weight 

 of these atmospheres must be in order to reach this maximum 

 of density. It is then found that the weight of the oxygen atmo- 

 sphere is only 0*56, and that of nitrogen 0*64, when that of the 

 hydrogen atmosphere is taken as the unit. 



If, therefore, we assume the presence of all these three gases 

 on the sun's surface, and if the influence of atmospheric motions 

 remain unconsidered, we shall find that the rays of light reach- 

 ing our eyes from the layer of hydrogen yielding a continuous 

 spectrum, had on their way to pass through so small a number 

 of glowing particles of nitrogen and oxygen that the absorption 

 thereby produced is infinitely small, and that therefore the pre- 

 sence of oxygen and nitrogen in the solar spectrum cannot be 

 ascertained, as we know is the case, by the appearance of dark 

 lines corresponding to these elements. 



Although the motion of gases will act in the direction of di- 

 minishing these great differences, still the existence of the chro- 

 mosphere shows the small effect of this motion, in consequence of 

 the great intensity of gravitation and the enormous elevation of 

 the gaseous mass (see formula 4). 



In order to explain by means of the above relation the absence 

 in the solar spectrum of the lines of two elements so widely dis- 

 tributed on the earth as oxygen and nitrogen, we must bear in 



