384 COSMIC PllILOXOrHY. [pt. ii. 



of tlio planets on a superficia] view, we find beneath this 

 apparent irregularity a marvellous symmetry of disposition 

 the explanation of which, though incomplete, is as far as it 

 goes in favour of the nebular hypothesis. The breaking up 

 of the zone of asteroids, though not fully explained, is seen 

 to have occurred in the only part of the system where such an 

 event, according to the hypothesis, was likely to occur. And 

 finally the geologic or meteorologic phenomena manifested by 

 the four planets whose surfaces have thus far been success- 

 fully studied, are just what the theory requires them to be. 

 The intense heat and furious volcanic activity of Jupiter 

 and Saturn, the extreme loss of heat and cessation of volcanic 

 activity upon the moon, the moderate temperature and habit- 

 able aspect of Mars, are alike deducible from the nebular 

 hypothesis. 



I doubt if such persistent agreement between deduction 

 and observation has ever been •witnessed in the case of an 

 erroneous or radically inadequate hypothesis. If the sole 

 ultimate test of a theory is that it reconciles the order of 

 conceptions with the order of phenomena, may we not say 

 that the theory of Kant and Laplace, having sustained the 

 repeated application of this test, may be accepted provisionally 

 as a true account of the past history of our system of 

 worlds ? It is true that the application of the test has not 

 yet been made exhaustive ; the verification is not yet 

 complete. Some of the interpretations above given are still, 

 as I have acknowledged, but partial ; and there are yet other 

 groups of phenomena with which I have not ventured to 

 meddle. To the various densities of the planets I have 

 alluded but incidentally ; and the various angular velocities, 

 as well as the order of distances formulated in the law of 

 Titius, still await an explanation. Besides which, the evi- 

 dence from the physical condition of the surfaces of Mercury 

 and Venus, Uranus and Neptune, and the moons of the fouT 

 outer planets, is not yet forthcoming. It would be asserting 



