COSMIC EXCHANGE OF MATTER AND LIFE 433 



It goes without saying that what is true of the meteors is 

 equally true of comets, since, as we have seen, they are pro- 

 bably one and the same thing. By whatever name we know 

 them, the result is a constant addition to every considerable 

 body of our own system, a steady taking up of cosmic material, 

 accompanied doubtless by a constant levy upon our system as, 

 through the long ranges of time, we come within the attractive 

 sphere of some dark or luminous sun. 



This unending exchange of matter and its larger aggregations 

 which we thus may picture, is not the sole way that the materials 

 of the universe might be equalised. There is another that is 

 probably going on under our eyes. It involves the infinitely 

 little as the other may engage the almost infinitely large. This 

 is the incessant radiation of particles from all incandescent 

 bodies. 



A candle, the arc of the electric-light, the sun too, probably 

 in brief, everything which glows is sending into the space 

 about it a shower of minute corpuscles of ultra-atomic dimen- 

 sions. Moreover, and quite independent of this, all highly 

 luminous bodies, and in especial the sun, are constantly driving 

 from them minute particles of matter under the'.pressure of light. 



Nothing at first seems stranger than that the immaterial 

 sunlight might exert a propulsive force. It is hard to think 

 that as it drives against the earth and other bodies it bears 

 down upon them with a certain energy. It was indeed a logical 

 inference from the ether- wave theory of radiation. Professor 

 Clerk Maxwell, who pointed out this necessity, mathematically 

 developed what this light pressure or force must be. He had a 

 rare combination of the mathematical that is, the essentially 

 analytic and the constructive or synthetic mind. In this 

 problem and in another he illustrated one of the astonishingly 

 rare instances in which mathematics has ever pointed the way 

 to scientific discovery. 



It is only two or three years ago that a Russian physicist, 

 Lebedeff, and almost simultaneously Nichols and Hull, in 

 America, by experiments of exceeding ingenuity, demonstrated 

 the existence of this pressure. It is extremely slight ; but it is 

 to be noted that the force it exerts will be in proportion to the 

 surface of the body upon which it acts. On the other hand, it 

 is evident that the powers of gravitation are in proportion to 



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