1 82 LIGHT SCIENCE FOR LEISURE HOURS. 



I do not consider it necessary to discuss Whewell's 

 theory that Jupiter and Saturn are intensely cold 

 planets, because it is professedly based on the theory 

 that they are formed of such terrestrial elements as 

 would, if in the same condition as upon the earth, 

 have the observed density of Jupiter and Saturn, and 

 that these substances, being further removed from the 

 sun, are correspondingly refrigerated. There is not a 

 line of direct reasoning, either a priori or a, posteriori, 

 in Whewell's chapters on the larger planets only 

 reasoning which depends on the assumptions which 

 had been made by those whom Whewell proposed to 

 controvert. In fact, his theory may be regarded, and 

 was probably regarded by himself, as merely a reductio 

 ad absurdum of the unreasoning faith of those who 

 had long held unchallenged the belief in the habit- 

 ability of all the planets. 



I proceed to indicate the leading arguments for the 

 theory that Jupiter and Saturn are still intensely hot, 

 noting first that I do not propose to discuss the details 

 of the various arguments l (which I have already done 

 elsewhere), and secondly, that the arguments are not 

 dependent one upon the other, but severally indepen- 

 dent, so that if any seem weaker than the rest, the 

 conclusion is not on that account invalidated, but the 

 weight of evidence only pro tanto diminished. It is 



1 I may, perhaps, be permitted to remark here, that the details of 

 many among the arguments here indicated will be found fully discussed 

 in the new edition of my treatise ' Saturn and its System.' 



