28 



Mr. G. J. Stoney on the Physical 



[Recess, 



drain off heat from this region. No heat passes beyond, except the small 

 quantity necessary to keep np the feeble escape from the lines of hydrogen, 

 sodium, and magnesium, and others of the same class, such as B, A, &c, 

 which are not only of a lower temperature, but are also few in number, if 

 we may deem those that fall within the visible part of the spectrum a 

 sufficient sample of the whole. Since, then, there is so much greater an 

 escape of heat from the upper layer of the iron atmosphere than from the 

 regions outside, there will exist a surface of minimum temperature near 

 the limit of the iron, beyond which there will be first a very slight recovery 

 and then a gradual fading off of the temperature. The observations of the 

 sodium lines indicate that this surface of minimum temperature which lies 

 near the outer boundary of the layer from which iron lines originate, can- 

 not be as hot as the flame of a Bunsen's burner. 



41. Within the iron atmosphere, on the other hand, there is a rapid 

 stream of heat directed outwards to supply the outpourings from near the 

 boundary of the iron atmosphere, as well as what is feebly dispersed by 

 lines such as those of hydrogen, sodium, and magnesium. Still further 

 down the stream becomes a torrent, as it has there to supply also the 

 lavish expenditure of heat by the multitude of lines more faint than the 

 iron lines, which are not only more numerous than lines of an intensity 

 comparable with the iron lines, but also each one of which discharges into 

 space a flood of heat proportioned to its exalted temperature, or, in other 

 words, to its faintness as a line in the spectrum. All this leads us to con- 

 clude not only that the temperature increases very rapidly within the iron 

 atmosphere, but that the rate of this increase becomes more and more pre- 

 cipitate as we descend. And this is in exact accordance with the intelli- 

 gence brought to us by the sodium lines, which, from being wingless, indi- 

 cate that the interval from the surface of the iron to the region where the 

 temperature first becomes comparable with that of the photosphere, is 

 both intensely hotter, and of trifling extent when compared with the vast 

 expanse from the surface of the iron up to the surface of the sodium 

 atmosphere. 



42. Molecules of magnesium have very nearly the same mass as mole- 

 cules of sodium. The two gases therefore rise to nearly the same height 

 in the solar atmosphere. Nevertheless the lines in the spectrum due to 

 magnesium present a very different aspect from those of sodium, into which 

 we must now inquire. The lines of sodium are narrow and sharp ; those 

 of magnesium broad and fringed, the borders being of the intensity that 

 Kirchhoff represents by the number 4. Now, the iron lines in their neigh- 

 bourhood are of intensities 5 and 6, which shows that the upper layer of 

 iron in which the iron lines take their rise may be distinguished into two 

 strata, the outer of which produces in that part of the spectrum lines of 

 intensity 6, while both together produce lines of intensity 5. To produce 

 a line of intensity 4, a third stratum below the layer in which iron lines 

 originate must be in action. Light reaches us from this third stratum in 



