210 



Prof. Schuster. 



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

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

 librium, and the spectroscopic appearance in the two cases will be 

 profoundly different. In reality an intermediate state probably is 

 arrived at, but there is good evidence to show that the state of con- 

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

 hydrogen stars. We know as a fact that there are powerful convec- 

 tion currents near the sun's surface. There is in consequence an 

 approach to a uniform distribution of matter and enormous differ- 

 ences 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 km. 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 spectroscopic 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 6 atmospheres, while that for the cooler calcium 

 vapour is about 3 atmospheres. With a gravitational constant 27 

 times as large as that of our earth, a difference of 3 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 Lyrse. 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- temperature 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 

 in the cooler and outer layers, while iron will be found more particu- 

 larly in the inner and hotter parts. The relative proportion of 

 different elements in different layers will be regulated partly by their 

 density, but to a great extent also by the total quantities present in 

 the star; for the different gases will not float on each other as liquids 

 might, but the density of each gas will increase steadily from the 

 surface to the centre. The chief difference, according to this view, 

 * • Astrophysical Journal,' vol. 3, p. 138. 



