The Groundwork of the Universe, &c. 69 



becomes white-hot, which implies that it is radiating all the 

 vibrations from the slow heat-rays up to the rapid waves of the 

 violet. Clearly from this example it is evident that the relative 

 temperatures of two pieces of heated iron can be gauged by the 

 length of contiiuious spectrum which they emit : the further this 

 spectrum extends towards the violet, the hotter the material 

 must be. 



By an examination of the stellar si)ectra, Lockyer was thus 

 able to catalogue the stars into various groups, the membei's of 

 each group having approximately the same temperature. 



Basing his views upon this experimental evidence, Lockyer 

 put forward what is known as his Meteoritic Hypothesis of 

 stellar evolution. He assumed as a beginning, a swarm of cold 

 meteorites travelling through empty space. As time passes, 

 gravitation will act upon this swarm, gradually drawing the 

 meteors nearer and nearer together; and the result of this process 

 will be a series of collisions between the flying bodies and the 

 consequent generation of heat. Imagine this process continued 

 for a considerable period, and it is obvious that in the next stage 

 of evolution we should find all the meteorites agglomerated 

 together into a loose mass Avith a moderately high tem})erature. 

 Still the force of gravity would act upon the material, tending to 

 condense the mass more and more ; and with the increasing 

 condensation and consequent increase in the number of impacts 

 between the particles, the temperature would continue to increase 

 until finally the whole of the material would be a flaming mass 

 of gas. At this point, it has reached its highest temperature ; 

 and its subsequent history will be one of radiation and cooling. 

 Slowly it falls in temperature, passing successively but in the 

 inverse order through all the temperatures through which it 

 travelled on the up-grade ; until finally it is reduced to a cold 

 globe, a dead star. 



After classifying the stars according to his results, Lockyer 

 found that in the dull, red stars, most of the ordinary metals 

 were present (as was proved \>y means of the spectroscope) in the 



