450 Mr. G. J. Stoney on the Hearing of 



before we could be spectators of them. It now appears that 

 some, at least, of them escape — or, to speak more accurately, the 

 marginal parts of some of them ; for it will probably be found, 

 if they can be submitted to examination with a sufficiently line 

 slit, that there is a central stripe of each blotted out by absorp- 

 tion, and appearing us a narrow dark line in the middle of each 

 bright line. We must also bear in mind that there may be other 

 lines emitted so narrow that they cannot cross the surrounding 

 atmosphere. 



100. Which lines then reached the earth ? Plainly those 

 which, as they were emitted by the incandescent gas, were 

 broader than the corresponding dark lines of the solar spectrum, 

 or at least than that part of the width of the latter which is due 

 to the absorption of those strata of the sun's atmosphere through 

 which we have to look in viewing the prominence. And as even 

 lines of the same gas differ from one another in this tendency to 

 spread out when heated, we sec at once how one hydrogen line, 

 C, could seem to Mr. Lockyer to extend further downwards in 

 the solar atmosphere than another, F*. The lower part of P was 

 no doubt effaced by the greater absorption of a hotter part of 

 the atmosphere telling with more effect upon F than upon C. 

 We must remember, too, that the ascending column may carry 

 with it gases from lower regions of the solar atmosphere to 

 heights not usually reached by them. Any rays which they 

 would then emit while in the gaseous condition would be un- 

 obstructed in passing out through the surrounding atmosphere ; 

 and such may be some of the faint lines seen by M. llayet. 

 However, such constituents would probably, for the most part, 

 soon cease to be gaseous, and augment the cloud by being cliilled 

 into a mist, or by hanging on the limits between their precipi- 

 tated and gaseous conditions in the way explained in § 71. In 

 any of these ways their spectral lines may be either enfeebled or 

 suppressed. 



101. Solar clouds may be of various kinds (sec §§ 09 and 

 71). But the light from a sufficiently attenuated mist of parti- 

 cles giving a continuous spectrum would have the property of 

 being so weakened in the spectroscope by being spread over the 

 whole length of the spectrum, that it might be easily overlooked 

 in such hurried observations as were made last Augustf. It ap- 

 pears probable, therefore, that the protuberance which was most 



* Comptcs Rcndus, October 26, 1868, p. 838, 



t Tims if a sail of sodium be burned in a candle-flame, though the so- 

 dium adds but little to the aggregate light of the flame, vet in a spectro- 

 scope with a sufficiently narrow slit the flame will give a background of 

 almost insensible faintness upon which the two sodium lines are brilliantly 

 projected. 



