146 INORGANIC EVOLUTION. [CHAf. 



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, between a hydro- 

 gen and a solar star lies in the more or less effectual mixing up of the 

 constituents. If we could introduce a stirrer into 7 Lyrse there can be 

 no doubt whatever that the low temperature lines of iron would make 

 their appearance, while, on the other hand, if we could stop all convec- 

 tion currents on the surface of the sun the hydrogen which now lies 

 under the photosphere would gradually diffuse out and give greater 

 prominence to its characteristic absorption lines." 



" In the face of the direct evidence of the absence of convection 

 currents in the hotter stars, it is not necessary for the purpose of my 

 argument to discuss why this is the case, but it can be seen that 

 diminished gravity, diminished density, and consequently increased 

 viscosity, will contribute to the effect, while effectual radiation will, 

 owing to the smaller density, take place more evenly through a thicker 

 layer of the envelope, so that the principal cause of convection currents 

 will also be much diminished." 



In replying to this objection of Dr. Schuster's I will first deal with 

 the convection currents and the tremendous temperature gradient 

 which Dr. Schuster postulates. In the sun, the seat of such convection 

 currents, according to him, while they are absent from y Lyrse they 

 are sufficiently powerful to cause a difference of 20,000 C. for each 

 100 kilom. in difference of level, or, as he otherwise puts it, a 

 difference of 100,000 for one second of arc. 



The eclipse photographs give no evidence of the rapid temperature 

 gradient in the sun supposed by Dr. Schuster. In the Indian series, 

 two successive photographs taken at intervals of about one second near 

 the beginning of totality differ inasmuch as the first includes a 

 stratum about 150 miles above the photosphere, which would be 

 covered by the moon when the second was taken (except for the effect 

 produced by irregularities in the moon's limb). Yet there is no great 

 difference in the spectra ; both contain arc and enhanced lines about 

 equally, and therefore indicate that the temperature changes can only 

 be small in a depth of 150 miles. In fact throughout a distance of 

 500 miles above the photosphere the spectrum indicates no change of 

 temperature of importance. 



We have got the facts then in the eclipse photographs, and find no 

 large spectral changes in a region where Dr. Schuster postulates a dif- 



