CHAP, XVIII.] REPLIES TO SPECIAL OBJECTIONS, 145 



'volume ikere, will be a reduction in density, and all the lines will be equally 

 tufecbled. But this is exactly what does not happen." 

 With regard to this statement Dr. Schuster writes : 

 " With this remark I cannot agree. The main fact to be explained 

 is the gradual displacement of hydrogen, which is predominant in the 

 hottest stars, by calcium, iron, and other metals. There are in my 

 opinion several causes at work which might produce that effect. A 

 glowing mass of gas may be either in thermal or in convective equili- 

 brium, and the spectroscopic appearances in the two cases will be pro- 

 foundly different. In reality an intermediate state probably is arrived 

 at, but there is good evidence to show that the state of convective 

 equilibrium is more nearly approached in our sun than in the hydrogen 

 stars. We know as a fact that there are powerful convection currents 

 near the sun's surface. There is, in consequence, an approach to a 

 uniform distribution of matter and enormous differences of temperature 

 in layers which are comparatively close together. Those who have not 

 given much attention to this subject will hardly realise the differences 

 of temperature brought about by convection currents. On the surface 

 of the sun the temperature gradient produced by convection currents 

 would be equal to 20,000 for each 100 kilometres difference in level, 

 so that an angular distance of one second of arc would correspond to a 

 difference of 100,000. Radiation and condensation will diminish this 

 gradient, but that it is very large is sufficiently proved by the spectro- 

 scopic evidence. Thus, according to the results of Messrs Jewell, 

 Mohler, and Humphreys,* the pressure in the reversing layer for hot 

 calcium giving the H and K lines is about six atmospheres, while that 

 for the cooler calcium vapour is about three atmospheres. With a 

 gravitational constant twenty-seven times as large as that of our earth, 

 a difference of three atmospheres can only mean a comparatively small 

 difference in level ; while, then, in the sun we must admit a more or less 

 effectual stirring up of the constituents together with an accompanying 

 rapid temperature gradient, the evidence is just the other way in the 

 case, of stars like 7 Lyrae. The spectrum of that star, according to 

 Professor Lockyer, contains only the high temperature lines of iron. 

 This means not only that the reversing layer is very hot, but also that 

 there are no rapid changes of temperature at different levels. It is 

 impossible to imagine this hot layer of gas ending abruptly ; it must 

 be surrounded by cooler matter, which cannot be iron, as the low tem- 

 perature lines of iron do not appear. In such a star there cannot be an 

 effectual mixing up of the constituents, and hence the layers of gas will 

 arrange themselves according to the laws of diffusion. It would follow 

 that hydrogen, being a lighter gas than iron, will be chiefly represented 

 * Astropkytical Journal, yol. iii, p. 138. 



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