COMETS' TAILS. 277 



and overcoming the mutual repulsion of their negative charges by their 

 mighty velocities, will clash together, like Lucretius' atoms, and unite 

 to form larger masses. But this aggregation must have an end. For 

 if, in the void of space, they are unable to get rid of their electric 

 charges, the potential of the growing mass must rapidly increase, since 

 the charge increases as the cube of the radius, being proportional to the 

 total number of particles, while the capacity for holding electricity 

 only increases as the radius itself. To put this in popular language, 

 each particle brings to the account the whole charge it can bear on its 

 surface; but in the mass, since electricity flies to the surface, only the 

 outer parts of those particles which are actually in the surface can be 

 useful in harboring the accumulating charge, and hence the electric 

 pressure rises. When it becomes intense enough to prevent fresh 

 particles from approaching, accretion will cease. Space will thus be 

 sown with masses of moderate size, formed irregularly, particle by 

 particle, in spite of repulsive forces. These are the meteorites which 

 blaze for a moment in the upper air, or in rare cases reach the earth 

 to puzzle philosophers with their porous structure. 



Another multitude of the particles will at last reach other suns. 

 For if in their wanderings they have united with others till they are 

 beyond the critical size, they will be drawn in, and raise the charge of 

 the bodies they reach, till they in turn discharge their streams into 

 space. 



In these we see the 'greyhounds' of the abyss, engaged in dis- 

 tributing the materials of the universe, forever busied in a cosmic 

 traffic by whose exchanges the stellar hosts are made more and more 

 alike in constitution, whatever may have been their differences in the 

 beginning. 



For those myriads which are fated to escape all visible suns, far 

 out in the 'flaming bounds of space' the Nebulae lie in wait, spreading 

 spider-like their impalpable webs across immeasurable breadths of sky. 

 Ever since the spectroscope showed that many nebulae are gaseous, and 

 yet shine by their own light, two problems have vexed the astronomers. 

 How can they be hot enough to send light to us, and yet be held 

 together against the expansive force of the heated gas, by the feeble 

 gravitation which such inconceivably diffuse masses can exert at their 

 borders? If they are really at a temperature of not less than 500° C, 

 so as to shine, or, indeed, if they are much above absolute zero, their 

 own gravitation should not be able to prevent their speedy dissipation 

 into space. 



Again, why do they show the spectroscopic lines of so few gases, and 

 those the lighter ones, such as hydrogen and helium? 



According to Arrhenius the nebulae are cold, with the cold of 

 empty space. Their light is due to the rain of negatively charged 



