1 66 LIGHT SCIENCE FOR LEISURE HOURS. 



gaseous matter was affected to any noteworthy extent 

 by the action of the sun's heat. Eaised from the solid 

 nucleus the vaporised particles passed first into the 

 coma, he imagined, and were thence carried off into 

 space to form the comet's tail. Others so far modified 

 Newton's views as to suggest that the vaporised matter 

 is not wholly carried off but partially re-precipitated 

 upon the head of the comet, just as the vapours raised 

 from the ocean are precipitated upon the earth in the 

 form of rain. 



We have seen that a comet diminishes in volume as 

 it approaches the sun. It will be noticed that both 

 the theories which have been described would account 

 satisfactorily for the observed decrease of volume. 

 But neither of them gives any satisfactory explanation 

 of the fact that a comet recovers its original volume as 

 it departs from the sun's neighbourhood. Newton, in- 

 deed, put forward certain views respecting the emission 

 of smoke from the nucleus during perihelion passage, 

 and he surmised that the true dimensions of the comet 

 might in this manner be veiled to a certain extent : 

 but this part of his theory has the disadvantage of 

 being almost unintelligible, besides being wholly in- 

 sufficient to account for the regular diminution and 

 increase which attend the approach and recession of a 

 comet. 



A theory has lately been put forward by M. Valz 

 which accounts for the variation of a comet's volume 

 by the supposition that the solar atmosphere exerts a 

 power of compression, which, varying with that atmo- 



