bh. v.] PLANETARY EVOLUTION. 395 



material medium, it meets with resistance ; it imparts motion 

 to the medium, and loses motion in so doing. If the body is 

 a planet like Jupiter, weighing a couple of septillions of tons, 

 and rushing along at the rate of eight miles per second 

 through an ether far lighter than the air left in an exhausted 

 receiver, the resistance will be inconceivably small, I admit. 

 Still there will be resistance, and long before the end of time, 

 this resistance will have eaten up all the immense momen- 

 tum of the planet. A Hindu, wishing to give expression to 

 his idea of the duration of hell-fire, said that if a gauze veil 

 were to be brushed against the Himalaya mountains once in 

 a hundred million centuries, the time required for thus wear- 

 ing away the whole rocky range would measure the torments 

 of the wicked. One marvels at such a grandiose imagination ; 

 but the realities of science beggar all such attempts at giving 

 tangible shape to infinitude. The resistance of an ethereal 

 medium may work its effects even more slowly than the 

 Hindu's veil, yet in time the effects must surely be wrought. 

 Either the planets are moving in an absolute vacuum — a 

 supposition which is incompatible with the transmission of 

 heat and light — or else the resistance of the medium must 

 tend to diminish their angular velocities. 1 



In the absence of any counteracting agencies — and, after 

 the cessation of the process above described, none such are 

 assignable — this loss of tangential momentum must ulti- 

 mately bring all the planets into the sun, one after another, 

 beginning with Mercury and ending with Neptune. Here the 

 concentration of matter appears to have reached its limit. 

 But what must now happen? 



Let us n^ts that the tangential momentum lost by the. 

 planet is lost only relatively to its distance from the sun. As 

 the planet draws nearer to the sun, its lost tangential 

 momentum is replaced, and somewhat more than replaced, by 

 the added velocity due to the increased gravitative forca 



1 See Balfour Stewart, The Conservation of Energy, p. 9(J. 



