144 



CHAP. XVIIL REPLIES TO SPECIAL OBJECTIONS. 



I NOW proceed to consider some less general objections. When I 

 brought the question of dissociation before the Royal Society in 1897, 

 in a discussion which I was requested to initiate, I pointed out that it 

 had been proposed to explain the spectral differences between such stars 

 as Bellatrix with its hydrogen and cleveite gases ; Sirius with its 

 tremendous development of hydrogen ; and our own sun and stars like 

 it with an atmosphere chiefly metallic ; by supposing that " the hydro- 

 gen and cleveite gases may from some reason or other escape from 

 among the metallic vapours and form an upper special atmosphere of 

 their own, in which, in consequence of its greater chemical simplicity, 

 the lines of these substances will become more important,"* and I 

 added, " But this argument is not philosophical, because we have no 

 right to assume such a change."! 



This remark, referring to a very special point, was .unfortunately 

 misheard, and Dr. Schuster in the discussion stated : 



" Had Mr. Lockyer confined himself to bringing forward his hypo- 

 thesis as one which is legitimate, consistent, and deserving of attention, 

 many of us would I think have agreed that he had made out a good 

 case. But he claims his theory as the only one which can explain the 

 facts, and dismisses as unphilosophical the only alternative which he 

 discusses." 



In spite of this misapprehension, however, Dr. Schuster's criticisms 

 are of great value, and I propose to consider them in this place and 

 reply to them as best I can. I may add that he expresses his concur- 

 rence with my system of classification ; and the necessity of a constant 

 appeal to laboratory experiment is insisted upon ; at the same time 

 he acknowledges that the investigation of the enhanced lines is "a 

 very material advance." 



In my paper I pointed out, in relation to stellar atmospheres, that 

 what we might expect to observe if we assumed the sun's temperature 

 to be increased would be vastly different according as dissociation did 

 or did not take place (see pp. 78-9). I said : 



' The only change which we can imagine on the usual hypothesis, as 

 resulting from the increase of temperature, is tJmt with the increase in 



* Proc. Soy. Soc., vol. Ixi, p. 202. 

 f Loc. cit. 



