﻿Observations of Comets. 459 



It must not be overlooked that on this supposition a formi- 

 dable difficulty presents itself in the insufficiency of the degree of 

 solar heat to which this comet was subjected for the conversion 

 of carbon into vapour. We cannot, for several reasons, suppose 

 that the solar heat was supplemented by the heat of chemical 

 action which it called forth. The suggestion, however, that car- 

 bon may possibly exist in an allotropic state in which it may be 

 much less fixed, and so capable of passing into vapour at a com- 

 paratively low temperature, is not perhaps inconsistent with our 

 positive knowledge of other elements. Even if this suggestion 

 were true to fact, a difficulty would remain ; for, so far as we 

 know, vapour in the non-luminous state, though it would not trans- 

 mit light of the refrangibilities which it would emit when heated, 

 would not give them back by reflection, so as to reflect light si- 

 milar to that which is peculiar to it in the luminous condition*. 



It may be well to consider for a moment the principal pheno- 

 mena which usually present themselves when a comet approaches 

 the sun. 



(1) Under the influence of the solar force the nucleus throws 

 out luminous jets which frequently assume the form of luminous 

 envelopes about itf. 



(2) The jets or envelopes rise, in the first instance, towards 

 the sun. 



(3) The envelopes are frequently separated from the head, 

 and from each other, by invisible spaces. 



(4) At the boundary of the head the envelopes behave as if 

 they had become subject to an intense force of repulsion from 

 the sun. 



(5) The matter of the envelopes appears to be driven from the 

 sun on all sides of the head, and in this way to form a hollow 

 conical tail. 



Though the feeble light of the comets hitherto subjected to 

 spectrum analysis has permitted them to be but imperfectly in- 

 vestigated, we have learned that the matter which emanates 

 from the nucleus, and is distinguished by a blue tint, gives a 

 light which the prism shows to be identical with that emitted 

 by the vapour of carbon. It is certain, therefore, that the light 

 which has the blue colour is not due to reflection from a cloud 

 of which the particles are too small to reflect the longer waves of 

 the less refrangible colours. 



* It might be possible that a spectrum of bright bands would be given by 

 a gas in a fluorescent state ; but the circumstance of the coincidence of the 

 cometary spectrum with that of carbon would remain unexplained. 



t Of these phenomena see a graphical account by Sir John Herschel, 

 ' Familiar Lectures on Scientific Subjects,' p. 115; also the " Account of 

 the Great Comet of 1858/' by G. P. Bond, which forms vol. hi. of the 

 'Annals of the Observatory of Harvard College.' 



