THE TWO COMETS OF THE YEAR 1868. 165 



respect to the volumes of comets" tails, inasmuch as the 

 apparent extent of these appendages varies according 

 to the atmospheric conditions (humidity, clearness, and 

 so on) under which the comet is observed, and also 

 according to the light-gathering power of the observer's 

 telescope. 



To return, however, to the theories which have been 

 formed respecting comets. 



It has been commonly admitted that the substance 

 of which comets are composed is either wholly or prin- 

 cipally gaseous. In no other way, it should seem, can 

 the remarkable variations of appearance which comets 

 present as they approach the sun or recede from him 

 be reasonably accounted for. 



Kepler held that comets are wholly gaseous, and 

 that they are liable to be dissipated in space by the 

 sun's action. He supposed that the process of evapora- 

 tion which thus led to the destruction of a comet was 

 carried on through the medium of the tail. It need 

 hardly be said that modern observations are completely 

 opposed to this view. Comets have been seen to return 

 again and again to the neighbourhood of the sun with- 

 out any apparent diminution of volume, although at 

 each return a tail of considerable length has been 

 thrown out. For a long time, indeed, it was thought 

 that Halley's comet was gradually diminishing in 

 volume ; but at the last return this magnificent object 

 had recovered all its pristine splendour. 



Newton held, on the contrary, that comets are partly 

 composed of solid matter. He supposed that only the 



