190 POPULAR LECTURES AND ADDRESSES. 



exhalations proceeding from the collision of 

 meteoric stones that Nebula:, and the heads of 

 Comets, show themselves to us, and he suggested, 

 at a former meeting of the Association, that ex- 

 periments should be made for the purpose of 

 applying spectrum analysis to the light which has 

 been observed in gunnery trials, such as those at 

 Shoeburyness, when iron strikes against iron at a 

 great velocity, but varied by substituting for the 

 iron various solid materials, metallic or stony. 

 Hitherto this suggestion has not been acted upon ; 

 but surely it is one the carrying out of which 

 ought to be promoted by the British Association. 



Most important steps have been recently made 

 towards the discovery of the nature of comets ; 

 establishing with nothing short of certainty the 

 truth of a hypothesis which had long appeared 

 to me probable, that they consist of groups of 

 meteoric stones ; accounting satisfactorily for the 

 light of the nucleus ; and giving a simple and 

 rational explanation of phenomena presented by 

 the tails of comets which had been regarded by 

 the greatest astronomers as almost pretematurally 



