434 THE WORLD MACHINE 



the mass of bodies. This has a curious consequence. The mass 

 of a body decreases in proportion to the cube of its diameter ; 

 but the surface decreases only in proportion to the square. It 

 follows, therefore, that with smaller and smaller bodies the re- 

 lations between the force of attraction and of light pressure 

 progressively alter ; gravitation decreases much more rapidly 

 than the force of light. It is evident, then, that there will come 

 a point where the pressure of light is actually greater than the 

 pull of attraction. At this point the particle has become ex- 

 ceeding small ; but it will then be shot away from a highly 

 luminous body, even against the tremendous gravitational pull 

 of the sun itself. 



This critical diameter, as it is termed, is reached with particles 

 less than a micron and a half thick, supposing them to have the 

 same density or specific gravity as water. A micron is the 

 ^-j^th part of an inch. The speeds which may be thus im- 

 parted to these particles are something enormous. A micro- 

 scopic body of matter of half the dimensions indicated will dash 

 from the sun at a rate of 500 kilometres per second. One 

 might imagine that with a constant decrease of the size of the 

 particle this speed might become practically infinite. As a 

 matter of fact it has a limit, which is perhaps eighteen times 

 500 kilometres ; that would be 9000 kilometres, or upwards of 

 5000 miles, per second. At such velocities a particle shot from 

 the sun would reach the earth after a few hours. 



The consequence of this pressure of light we are only just 

 beginning to perceive. To be sure, the quantity of matter in- 

 volved is extraordinarily slight. On the other hand, the out- 

 pour is enormous. Und die Zeit ist unendlich lang. It is an 

 ingenious suggestion of Arrhenius, from whose paper these 

 interesting details are drawn, that we may have here the solution 

 of one of the outstanding riddles of cosmical physics. Perhaps 

 this is the origin of the meteorites. 



It is very striking that the meteorites seem to consist of 

 minute particles in relatively loose combination. They are 

 rather porous, and seem to be built up particle by particle. 

 When they are melted down the result is a solid and vitreous 

 mass, a very different substance from the pumice-like meteorite 

 which reaches the earth. In their wanderings through space 

 we may conceive these particles coming into collisions sufficient 

 lightly to fuse them together. Once such a nucleus was formed 



