September 1, 1898.] 



KNOWLEDGE 



177 



different stars do not vary equally and simultaneously is 

 obvious, and I cannot therefore regard it as inconceivable 

 that in this instance the H line may be truly missing. 

 The existence of two lines on the one side and of four on 

 the other render it, however, very improbable. 



The other suggestion is, that we have to do with two 

 distinct bodies, and that the body giving rise to the bright 

 hydrogen lines lies behind that surrounded by the dense 

 calcium vapour. This, again, for obvious reasons is very 

 improbable. It remains only to confess that at present I 

 see no solution of the difficulties before us, and I think 

 that it is from the intricacy of the problem, and not from 

 any oversight, that the spectrum of Mira Ceti has received 

 so little comment hitherto. Spectroscopists are the more 

 indebted to Miss Gierke for her suggestion as to the next 

 observations to be secured. The more perplexing the 

 problem before us is, the more complete and fruitful will 

 its solution be when we at length possess the key. 



,., E. W. Maunder. 



ABSORPTION IN THE SUN. 

 To the Editor of Knowledge. 



Sir, — My article on " The Sun as a Bright-line Star," 

 in the last number of Knowledge, was written without 

 prepossessions for or against any general theory of solar 

 absorption. My object was simply to draw some inevitable 

 conclusions (as they appeared to me) from certain well- 

 ascertained facts. If the constitution of the " reversing- 

 layer " is largely implicated, the responsibility rests with 

 the facts, not with my infei-ences from them. But since 

 you, sir, have, in a courteous editorial note, expressed 

 doubts as to their validity, it is necessary to reconsider 

 them. 



Following your example, I will take the K-line alone. 

 This quality of radiation comes to us bright from faculfe 

 on the disc of the sun. The vapour emitting it is then 

 hotter than the photosphere : it is, to a still greater degree, 

 hotter than the absorbing stratum, which, because cooler 

 than the photosphere, stops the whole of its light of the 

 same wave-length as K. I say the whole, because the 

 remnant of K-illumination in the solar spectrum comes 

 from the absorbing layer itself, not from the photosphere. 

 The screening action is complete, but the screen is self- 

 luminous. Between this absorbing layer and the faculse 

 there is, accordingly, a wider difference of temperature 

 than between it and the photosphere. If, then, it arrests 

 all the photospheric light agreeing with K in refrangibility, 

 it should, a foriiari, if situated at a higher level than the 

 faculii', arrest all the K-light derived from them. The 

 fact that, to all appearance, none of it is arrested, evidently 

 relegates the absorbing calcium-stratum to a lower level. 



Another circumstance seems equally demonstrative to 

 the same effect. If the chromosphere were enveloped in 

 vapour absorbing and emitting K-light, a line drawn 

 through it to the edge of the sun should be many times 

 longer than a line drawn to the centre of the sun. Hence 

 any action exercised by it upon faculffi should be vastly 

 intensified as regards our spectroscopic view of chromo- 

 sphere and prominences. Nevertheless, the K-line is 

 scarcely less brilliant in them than in faculie. Moreover, 

 a calcium-stratum profound enough to render the difference 

 in question negligible, is totally inadmissible. Even if it 

 had a depth equal to the sun's radius, the ratio of the 

 lines penetrating it to the centre and limb of the sun would 

 still exceed one-half; and in this case, the rays emitted by 

 it, showing bright ofi" the sun, should invariably and every- 

 where attain a height above the limb of 15'. 



Passing to the spectrum of Mira, our editor maintains 

 that the absence from it of the bright H-liue of hydrogen 



" cannot be taken as proving that the cool calcium-vapour 

 is above the glowing hydrogen." The absorption by the 

 calcium H of the nearly coincident hydrogen-ray may, 

 we are told, be entirely due to difference of temperature. 

 But how can one gas be cooler than another if they are 

 intermixed, as it is assumed they are, in one and the same 

 locality ? Yours faithfully, 



A. M. Clerke. 



[If the gases which absorb the light of the sun's photo- 

 sphere formed an atmosphere about the sun, it would be 

 impossible to conceive of two kinds of vapour at different 

 temperatures existing in the same region. But we may 

 feel sure that the vapours present in the chromosphere and 

 corona of the sun do not form an atmosphere in which 

 the gases rest in equilibrium or in layers upon one another. 

 With solar gravity equal to 27j times terrestrial gi-avity, 

 such an atmosphere, even if composed of hydrogen, could 

 not extend to a height of a few hundred miles without 

 being reduced in density so as to be quite negligible. 

 Thus it may be demonstrated that at a temperature of 

 2457' Cent, hydrogen would be reduced to a millionth of 

 a millionth of its density at the lower level on rising 

 through a height of 270 miles. We must therefore con- 

 clude that the molecules of the gases above the photosphere 

 are moving in long free paths, having been evaporated from 

 solid or liquid particles falling from a cooler region, or that 

 they are derived from prominence streams rushing upward 

 from a lower level ; in either case we may have molecules 

 at the same level at very different temperatures, because 

 they are derived from different sources. 



I admit that there is an appearance of stratification in 

 the lower and hotter regions of the sun immediately above 

 the photosphere, but I would urge that this can only be 

 an apparent stratification due to the effect of temperature 

 in modiiying the general physical condition and the 

 spectrum given out, and that in the stars as well as in the 

 sun there must be a complete mixture of materials ; though 

 as we approach the heated centre there will be regions 

 where some of the materials condense into the liquid or 

 solid state, while the rest remain in a state of vapour. 



It seems highly probable that similar molecules will 

 always give out or absorb the same wave-lengths under 

 similar conditions, and that they will always condense and 

 vaporize at the same temperature, and at first sight it 

 might seem necessarily to follow that the driving into 

 vapour of different materials would always succeed one 

 another in the same order as we approach a highly- 

 heated body ; but there are at least two possible ways 

 in which a variation of the order may be accounted 

 for. 



1. Differences in the mass of stars would cause matter 

 condensed in the coronas surrounding the stellar photo- 

 spheres to approach the heated centre with different 

 velocities, and before complete evaporation falling particles 

 would be carried to different depths or isothermal regions. 



2. In stars at different temperatures we may have very 

 dift'erent intensities of the explosive action which, in the 

 sun, is continually driving the matter of which the promi- 

 nences and the corona are composed away from the 

 centre, to cool in the outer regions and fall again. It 

 seems probable that the mass of the star remaining un- 

 altered, the greater the heat the greater would be the 

 velocity of the up-rushes, and incandescent material would 

 be carried into colder regions before it ceased to shine. 



In addition to these two causes of variation, stars may 

 not all consist of similar materials, or of materials mixed 

 in the same proportion ; but it is evident that this last 

 possible cause of variation will not account for the differ- 

 ence between the spectra of Mira and the sun. 



