CHAPTERS ON THE STARS. 27 



relatively to each other, which come into constant collision. Their 

 velocity is such that at each collision heat and light are produced. 

 In the language of our progenitors, who in the absence of matches used 

 flint and steel, they 'strike fire' against each other. The idea of such 

 a process originated with Prof. P. G. Tait, in an attempt to explain 

 the tail of a comet, but it was elaborated and developed by Mr. Lock- 

 yer in his work on the 'Meteoritic Theory.' 



The objections to this theory seem insuperable. A velocity so 

 great, at such a distance from the center of the nebulas, would be in- 

 compatible with the extreme tenuity of these objects. Every time 

 that two meteors came into collision they would lose velocity, and, 

 therefore, if the mass was sufficient to hold them from flying through 

 space, would rapidly fall toward a common center. The amount of 

 light produced by the collision of two such objects is only a minute 

 fraction of the energy lost. The meteors which fall on the earth are 

 mostly of iron, and, were the theory true, numerous lines of iron 

 should be most conspicuous in the spectrum. But the fact is that in 

 the great number of these objects there is but a single bright line, 

 which does not seem to correspond to the line of any known substance. 

 The supposed matter which produces it has, therefore, been called 

 nebuhim. 



