82 INORGANIC EVOLUTION. [CHAP. IX. 



perature. It is most natural to suppose that these increasing tem- 

 peratures produce increasing simplifications. 



Dealing, then, with the changes which we can now study in stellar 

 bodies from the temperature of the sun upwards, we have the series of 

 spectral changes on which the new chemical classification (Chapter 

 VII) has been based. 



Now if dissociation is not the cause of these changes where are we 

 to look for one equally simple and sufficient 1 



It is quite clear that the phenomena to be observed with every 

 increase of temperature, that is in a series of stars with spectra 

 gradually extending more and more into the ultra-violet, must be 

 vastly different if the elements are dissociated from what they would 

 be if the elements remained unchanged. 



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

 resulting from the increase of temperature, is that with the increase 

 in volume there will be a reduction in density, and all the lines will be 

 equally enfeebled. But this is exactly what does not happen. 



It may be said that in consequence of a more exalted temperature 

 in the hottest stars the hydrogen and cleveite gases may, for 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. But this argument is not philosophical, because we 

 have no right to assume such a change. These gases already exist in 

 the sun and give us no traces of their existence at any great height 

 above the chromosphere ; the gas that does exist in these elevated 

 regions is one about which we know nothing, so far, terrestrially, and 

 of which no trace has yet been found in the spectrum of the hottest stars. 



I hold, then, that the stars more than justify my appeal to the law 

 of continuity ; their verdict is that, as in all previous human expe- 

 rience, a higher temperature brings about simplifications, and it is not 

 strange that as our horizon is expanded by new work we find the 

 same laws in operation. We have, in fact, in these phenomena the 

 work of dissociation carried on before our eyes in the hottest stars, to 

 a point not reached anywhere else, and the stars also tell us that this 

 is possibly beyond our laboratory possibilities, for the highest tempera- 

 ture I have employed only carries us to the heat level of y Cygni, in 

 which star the cleveite gases, if visible, give only very faint traces. We 

 are thus brought finally face to face with the fact that iron is a com- 

 pound into the ultimate formation of which hydrogen, or the cleveite 

 gases, or both, may possibly enter. 



