158 LIGHT SCIENCE FOR LEISURE HOURS. 



lively. They cannot be identified with the lines seen 

 in the spectra of any known terrestrial gases. 



Of whatever gases the nucleus is composed it appears 

 that conditions wholly different from any with which 

 we are familiar on earth prevail in this, and doubtless 

 in all other comets. The gases which form the nucleus, 

 though self-luminous, are probably not incandescent. 

 Remembering that comets are luminous when situated 

 far out in space beyond the orbit of our own earth, we 

 are prevented from assuming the existence of an inten- 

 sity of heat (due to solar action) sufficient to account 

 for their inherent light. And if the light of a comet 

 were due to a state of incandescence in the component 

 gases, there would be a rapid consumption of the 

 substance of the comet, and we should be quite 

 unable to account for the fact that Halley's comet 

 has continued to shine, with no appreciable loss of 

 brilliancy, for upwards of three hundred years. We 

 seem forced therefore to surmise that the gases which 

 form the substance of comets owe their light to a species 

 of phosphorescence which is independent of the comet's 

 temperature, or else to some electrical properties the 

 nature of which it would not be easy to divine. 



Our perplexity is increased when we see the gases 

 which form the nuclei assuming either the liquid or 

 the solid form in the outer part of the coma. The 

 change from gaseity to liquidity or solidity is an evi- 

 dence of loss of heat, whereas one would expect the 

 outer part of the coma, which is exposed to the full 



