XVIII.] REPLIES TO SPECIAL OBJECTIONS. 147 



fererice of 100,000 0. Are we to take this value as the temperature 

 of the sun's photospheric level ? If so, how does Dr. Schuster reconcile 

 it with the values obtained by all the recent workers who make it less 

 than 10,000 C. 1 and even with 'Homer Lane's 28,000 1 



Surely the facts show that there are not-, in the sun, such tremen- 

 dous convection currents as are demanded on Dr. Schuster's view. 



Professor Schuster refers to the conclusion drawn by Messrs. Jewell 

 and others as to the pressure' of hot and cold calcium in the reversing 

 layer. His reference shows that he agrees with my view that we are 

 dealing with different molecules, but I wish to 'remark that I think we 

 must not be too hasty in accepting the conclusions to which he refers, 

 for the reason that the eclipse photographs do not tally with them at 

 first sight. In these photographs (1898) the K layer reached a height 

 of 6,000 miles ; the A 4226'96 layer only 2,000 miles. This suggests 

 that cool calcium falls and is dissociated at the bottom. It certainly 

 does not mean that there is a layer of cooler calcium at a higher eleva* 

 tion and at less pressure surrounding a hotter one at a lower elevation 

 and higher pressure. 



The evidence on which it is assumed that convection currents are 

 absent from the hotter stars likejy Lyrse of decreasing temperatures 

 does not appear to be conclusive. But let us assume it. 



The absence of cool iron lines only shows that we are in a region 

 of higher temperature than in the sun. May there not still be a rapid 

 temperature gradient, from " high " to " very high " temperature 

 instead of from " low " to " high " as in the sun ? But in any case, a 

 mere stirring up of 7 Lyrse would not make its spectrum like that of 

 the sun. Such stirring up could only introduce the cooler lines of iron 

 if the proto-iron were by that process driven out into the cooler 

 regions, where it might become iron and so produce cool iron absorp^ 

 tion lines in the spectrum of the star. But it by no means follows 

 that these cool iron lines would be as strong as in the solar spectrum, 

 for we know that the amount of absorbing proto-iron is only small. 

 Moreover, this process of stirring would hardly reduce the intensity of 

 the hydrogen lines. 



A reduction of temperature, however, furnishes us with a sufficient 

 explanation of the changes observed in passing from such a star as 

 7 Lyrse to one like the sun ; the cool lines of iron would appear as a 

 matter of course, and such lines would become stronger if iron can be 

 formed at the expense of the hydrogen. 



If we take the converse view, and suppose the postulated convec- 

 tion currents in the sun to be stopped, I do not see how such a condi- 

 tion of things would result in changing the present spectrum of the 

 sun into a spectrum like that of 7 Lyrae. We have not only to ex- 



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