234 THE NEW KNOWLEDGE. 



surface to its weight would be enormous, it would be al- 

 most all surface. Now the greater the surface the greater 

 the effect of the light pressure and hence without going 

 down, by any means, into infinitesimals the process of divi- 

 sion carries us to a particle so fine that the light pressure 

 will exactly balance its weight. This is the case with a 

 particle of earth 1/100,000 of an inch in diameter. Such a 

 particle would neither be attracted nor repelled from the 

 sun, for the sun's pull upon it is exactly balanced by the 

 repulsive force of the sun's light. If the particle is smaller 

 still it is repelled from the sun and, in fact, if the particle 

 is exceedingly small the light-push may enormously exceed 

 its weight. But 1/100,000 of an inch is not in itself very 

 small. We know many bodies smaller than this, without 

 considering the atoms and corpuscles of preceding chapters. 



One of the greatest mysteries in astronomical science has 

 been the comet's tail and why it points away from the sun. 

 The facts are generally known. The tail of a comet may be 

 any length up to 100,000,000 miles. It develops and grows 

 larger as the comet approaches the sun, proceeding back 

 from the direction of motion of the comet like the smoke 

 from a steamer; but unlike the steamer-smoke, as the comet 

 rounds the sun and flies away; the tail now precedes the head. 

 It is precisely as though there was a strong repulsive wind 

 blowing away from the sun and sufficing to keep the comet's 

 tail pointed away from it. The nature of this "repulsive 

 force has been the mystery of astronomy. Now, however, 

 vve may consider ourselves acquainted with its cause. 



The whole comet has a spectrum practically identical 

 with that of a bunsen burner consuming ordinary gas, 

 and, hence, evidently consists of hydrocarbons. In addi- 



