46 INORGANIC EVOLUTION. [CHAP. 



gases, hydrogen and the cleveita gases and carbon compounds driven 

 out of the meteorites as a result of the heat produced by the collisions ; 

 and to a less extent to the low temperature lines of some of the chemical 

 metallic elements known to exist in meteorites. 



We have then to deal with the colliding particles of the swarm 

 and the permanent gases given off and filling the interspaces. The tem- 

 perature is relatively low ; since gases may glow at a low temperature 

 as well as at a high one, the temperature evidence depends upon the 

 presence of cool metallic lines and the absence of the enhanced ones. 



The nebulae, then, are relatively cool collections of some of the per- 

 manent gases and of some cool metallic vapours, and both gases and 

 metals are precisely those I have referred to as writing their records 

 most visibly in stellar atmospheres. 



If the nebulae are thus composed, they are bound to condense to 

 centres, however vast their initial proportions, however irregular ths 

 first distribution of the cosmic clouds which compose them. Each 

 meteorite, the motion of which is stopped by collisions, must at once 

 fall to the centre of gravity of the swarm. 



Each pair of meteorites in collision puts us in mental possession of 

 what the final stage must be. We begin with a feeble absorption of 

 metallic vapours round each meteorite in collision ; the space between 

 the meteorites is filled with the permanent gases driven out further 

 afield, and having no power to condense. Hence dark metallic and 

 bright gas lines. As time goes on, the former must predominate, for 

 the whole swarm of meteorites will then form a gaseous sphere, with a 

 strongly heated centre, the light of which will be absorbed by the 

 exterior vapour. 



As condensation goes on, the temperature at the centre of condensa- 

 tion always increasing, all the meteorites of the parent swarm in time 

 are driven into a state of gas. The meteoritic bombardment practically 

 now ceases for lack of material, and the future history of the mass of 

 gas is, speaking generally, that of a cooling body, the violent motions 

 in the atmosphere while condensation was going on now being replaced 

 by a relative calm, producing a quiescent reversing layer the observa- 

 tion of which alone enables us to define the temperature of the star. 



The temperature- order of the group of stars with bright lines as 

 well as dark ones in their spectra, has been traced, and typical stars 

 indicating the spectral changes have been as carefully studied as those 

 in which absorption phenomena are visible alone, so that now there are 

 very few breaks in the line connecting the nebulas with the stars on the 

 verge of extinction. 



We find ourselves here in the presence of minute details exhibiting 

 the workings of a law associated distinctly with temperature ; and 



