234 SCIENCE FROM AN EASY CHAIR 



tion affecting a comet may be such that the return journey 

 never occurs. They may be such that the comet goes on 

 indefinitely travelling away from our sun, until he is 

 caught by some other star, and his orbit changes its shape, 

 with the new sun as attracting centre. These are the 

 " wandering comets " as distinct from the " periodic 

 comets," which have been shown to conform to Halley's 

 scheme of their movement and recurrence. 



And now some one will ask, perhaps impatiently, 

 " What, after all, is a comet ? " We have seen that many 

 are continuously, and others casually, members of the solar 

 system. What do they consist of? Spectrum-analysis 

 shows that they consist chiefly of the chemical element 

 carbon. 1 Though they have weight, and are attracted by 

 the sun, yet they seem to be for all their size and terrifying 

 shape and glare incredibly light and airy things. Herschell 

 declared that the tail of a big comet probably consisted of 

 but two or three pounds of solid matter diffused, rarefied, 

 and luminous. And the head or nucleus certainly does not 

 weigh many hundreds of tons. In the eighteenth century 

 astronomers observed a comet pass right in among the 

 moons of the planet Jupiter. You might expect the 

 moons to be terribly knocked about by such an impact. 



1 I am indebted to Mr. Rolston, of the Solar Physics Observatory, 

 South Kensington, for some information on this matter. 



Generally speaking, it appears that the spectra of these bodies 

 indicate carbon in some form as the principal constituent. 



As to the particular form of carbon, there is still a considerable 

 doubt, so much that, in describing the spectrum of Morehouse's 

 comet, Professor Frost says (Astrophysical Journal, xxix., p. 59, 

 1909) : " We avoid the still unsettled question of the ' carbon ' bands 

 (of the so-called ' Swan ' spectrum) which have been so often ascribed 

 to a hydrocarbon, specifically acetylene, and we use for them the 

 simple designation 'carbon.'" 



In addition to this " carbon " there is the cyanogen spectrum 

 present in most cases. 



Sodium and iron have been detected in the spectra of some few 



