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 distributing 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 Nebulæ lie in wait, spreading spider-like their impalpable webs across immeasurable breadths of sky. Ever since the spectroscope showed that many nebulæ 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 nebulæ are cold, with the cold of empty space. Their light is due to the rain of negatively charged
