292 R. Meldola—Bright Lines in the Solar Spectrum. 
the sun’s atmosphere, rising as a necessary consequence, into 
regions which are cooler than that stratum which is cool enough 
to reverse the spectral lines of those metals having the smallest 
molecular mass, viz: Na, Ca and Mg.* Professor Draper’s 
suggestion that the enormous thickness of incandescent oxygen 
may overpower the light of the photosphere, can onl 
good, when considered in connection with this hypothesis, if 
the temperature of the upper portions of the oxygen atmosphere 
does not differ to any great extent from that of the lower and 
hotter portions. When, however, we bear in mind the com- 
paratively low vapor density of oxygen, and consider at the 
same time to what an enormous height the hydrogen atmosphere 
extends, it appears probable that the height reached by oxygen 
would be such that the temperature of the upper portions of 
this gas would be considerably lower than that of the subjacent 
layers; so that any excess of radiation over that of the photo- 
sphere given out by the hottest portions of the incandescent 
oxygen would be obliterated by the absorption of the cooler 
portions above. 
[The same reasoning can be applied if we suppose that the 
temperature of the oxygen falls off at some particular level ; 
so that above this boundary the state of molecular aggregation 
of the gas corresponds to Dr. Schuster’s ‘‘compound line” spec- 
trum, while below this boundary the greater heat of the gas 
resolves its molecules into the atoms giving the ordinary line 
_ spectrum. The effect of this state of affairs is practically the 
same as would be brought about by annihilating a certain por- 
tion of the upper oxygen layers, since the two different molec- 
ular states of the gas give totally dissimilar spectra. We are 
thus reduced to an oxygen atmosphere of smaller extent, and 
the foregoing reasoning obtains. : 
ngstrém suggested + that the non-appearance of the lines 
of oxygen and nitrogen in the solar spectrum might be ac- 
counted for by supposing that, at the temperature of the sun, 
the specific absorptive power,of these gases may be insufficient 
to reverse their spectra. This view, however, equally fails to 
account for the brightness of the lines in question. 
Let us now make the not improbable assumption that the 
temperature of the sun’s nucleus, photosphere, and reversing 
layer is so great that dissociation is perfect throughout these 
* Stoney has shown (Proc. Roy. ii, p. oa 
when present in only small uaa aes aoe dake aoa i ets sare full 
wile vesaiae (SOC Oeaee le spectre Solaire, Upsal, 1869, p. 37) that it is 
emu ire, 1 P- me 
“trés-probable que 1 i , les 
ses farnarea e Vox température élevée du soleil - suffit pas pour 
occasioner de raies obscures dans le spectre 
i y exist in the corona. 
