30 



Mr. G. J. Stoney on the Physical 



[Recess, 



of hydrogen. The temperature of the sun's atmosphere ahove the surface 

 of the iron is too low to dilate hydrogen lines. The breadth, therefore, of 

 the black part of the hydrogen lines must be due to the quantity of this 

 element which is to be found in the interval between the outer boundary 

 of the iron and that situation in which the temperature first becomes 

 too high to appear black when projected against the brightness of the 

 photosphere. This interval is small in the part of the spectrum where the 

 line C occurs ; at the line F it extends through a considerable part of the 

 thickness of the layer that gives out iron lines ; at the hydrogen line near 

 G it extends quite through this layer ; and in the situation of the fourth 

 hydrogen line it extends much further down. But even in the least of 

 these intervals there is enough of hydrogen to give a very sensible breadth 

 to the line C. This quantity must be very considerable ; as also must the 

 quantity which can produce, in the hotter regions below, the fringes which 

 border all the hydrogen lines. To recapitulate, — the width of the hydrogen 

 lines, the wings that fringe them, the intense line in the sun's spectrum 

 which corresponds to a faint hydrogen ray, and the height to which 

 hydrogen can support traces of other gases, and more especially the vestige 

 of sodium in the solar atmosphere, all testify to the abundance of this 

 element. 



45. The sodium lines D are an open channel through which heat is 

 poured from a very hot region into that immense upper expanse of the 

 sun's atmosphere which is tenanted by sodium, magnesium, and hydrogen 

 alone. This is not the case with the magnesium lines of the group 5, nor 

 with the four hydrogen lines. These all stop heat before it has travelled 

 to any great distance, by reason of the great abundance of hydrogen, and 

 by reason of the specific opacity of magnesium for the rays b, and its 

 quantity, which, though small, is immeasurably greater than the quantity 

 of sodium. And on a different account, the same may be true of the 

 faint rays of the spectra of sodium and magnesium. Two such mag- 

 nesium rays were observed byKirchhoff of wave-lengths 44*92 and 46*06; 

 and Iluggins has recorded three faint pairs of sodium lines, of wave-lengths 

 51*6, 56*9, and 61*6, and a nebulous band at 49*9. It is not yet fully 

 ascertained whether there are lines in the solar spectrum answering to any 

 of these rays. If there are such lines, they are faint. Now, if it shall 

 prove that no such lines can be detected, it will indicate that heat from 

 beneath of these wave-lengths passes without sensible diminution through 

 the cool parts of the sun's atmosphere and therefore does not heat them • 

 and if it be found that they give rise to faint lines, this faintness is to be 

 attributed to but little of the heat despatched from hot regions being en- 

 tangled in its passage outwards. Similarly the heat which is so trans- 

 mitted through the wings of conspicuous lines crosses with little obstruc- 

 tion the colder regions above ; since at the temperatures that there prevail 

 few of the periodic times of the atomic orbits deviate sufficiently from 

 those central periodic times which correspond to the middles of the lines. 



