Page:Popular Science Monthly Volume 18.djvu/530

514 atmospheric electricity. It has been ascribed to chemical action in vegetation, without any basis of proof; also with more reason to evaporation and to friction of solid and liquid particles against bodies on the surface of the earth. But friction fails to account for the fact that electrical displays, other than auroral ones, occur only during heavy precipitation of hail or rain, except in mountainous districts.

Deferring for the present what appears to be a probable account of the generation of atmospheric electricity, we may profitably review certain facts established by observation and experiment.

It is a matter of common observation that a small ascending jet of water is resolved into drops, which describe widely divergent trajectories. By reason of the different velocities and directions of motion of the individual drops, they come into frequent collision with one another and then rebound. The influence of electricity on the recoil of the drops after collision is most marked and interesting. About two years ago, Lord Rayleigh read a paper before the Royal Society on this subject, setting forth the results of his experiments.

When the ascending jet is strongly electrified, the drops do not collide, because of their mutual repulsion, arising from their charge of electricity of like sign. But with a very feeble charge the drops coalesce upon impact, and the breaking up of the stream is thereby much lessened. The rubbing of a glass rod across the sleeve in the vicinity of the jet suffices to prevent the rebound after collision; as also does the current from a single Grove element.

Further experiments, in which one of two contiguous jets was electrified, proved that the coalescence was due to a slightly different degree of electric tension in the impinging particles of water. In such case their attraction and coalescence are determined by static induction, the resulting force of which is always attractive; and with slight electrification the inductive effect always prevails over the repulsion due to like charges, when the charged bodies are brought near together. Hence strongly electrified particles do not actually collide, but are kept apart by electrical repulsion; while those feebly charged approach within the charmed circle where the attraction of static induction determines their collision and coalescence.

The bearing of these facts upon precipitation of aqueous vapor is obvious. Innumerable globules of water, feebly charged to different potentials, inevitably collide and coalesce into drops, which descend by gravity. A slight amount of electricity in the atmosphere is therefore favorable to aqueous precipitation, while a higher degree of electrical excitation is unfavorable to rapid condensation.

It is important in this connection to point out another conclusion bearing upon condensation of vapor—a conclusion reached by applying mathematics to the theory of electrified spheres or drops of water. An explanation of terms is essential to an understanding of the reasoning.