158 INORGANIC EVOLUTION. [CHAP. 



compounds, say chloride of sodium, that is common salt, and oxide of 

 iron, that is iron-rust, we produce the simpler substances of which they 

 are composed by heat, and we further have no difficulty in recognising 

 the fact that chlorine and sodium in one case, and oxygen and iron in 

 the other, must have existed before their compounds, common salt 

 and iron-rust, could be formed or associated. Water is split into 

 hydrogen and oxygen at a high temperature, so that there is a tem- 

 perature above, which the two. gases would remain in contact but 

 uncombined; when the temperature falls water is produced. Disso- 

 ciation, therefore, in all its stages must reveal to us the forms the 

 coming together of which has produced the thing dissociated or broken 

 up by heat. If this be so, the final products of dissociation or breaking up 

 by heat must be the earliest chemical forms. Hence if the various stars 

 behave like the various geological strata in bringing before us a pro- 

 gression of new forms in an organised sequence, we must regard the 

 chemical substances which visibly exist in the hottest stars which, so 

 far as we know, bring us in presence of tsmperatures higher than any 

 we can command in our laboratories, as representing the earliest 

 evolutionary forms. 



I have said if. Now do the stars from the hottest to the coldest 

 present us with a progression of new forms as the geological strata do 

 from the. oldest to the newest ? 



The preceding pages enable us to answer this question fully. On 

 p. 47 I indicated how, in cosmical evolution, we deal with a continuity 

 of effects accompanied by considerable changes of temperature ; from 

 the gradual coming together of meteoritic swarms until eventually we 

 have a mass of matter cold and dark in space. The various stars 

 which represent the different changes have been got out and have, in 

 fact, been arranged along a so-called temperature curve. As we 

 ascend one branch of this curve the stars get gradualty hotter and 

 hotter till ultimately at the top we find the hottest stars that we know 

 of. Then on the descending branch are represented the cooling bodies, 

 and finally they come down in temperature until we reach that of a 

 dark world like the companion of Sirius, of our own moon, and the 

 planet in which we dwell. 



Thanks to the recent work, we can now deal with all these bodies 

 in special relation to their chemistry. No doubt the record will be 

 made more complete as time goes on and other workers come into the 

 field ; but it is already complete enough for my present purpose, for 

 the story is one of changes of chemical forms from one end to the 

 other. 



When the photography of stellar spectra work was begun our 

 knowledge was so incomplete that a continuous chain of chemical facts 



