514 THE POPULAR SCIENCE MONTHLY. 



atmospheric electricity. It has been ascribed to chemical action in 

 vegetation, without any basis of proof ; also with more reason to evap- 

 oration 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 trajec- 

 tories. By reason of the different velocities and directions of motion 

 of the individual drops, they come into frequent collision with one an- 

 other 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 col- 

 lide, because of their mutual repulsion, arising from their charge of 

 electricity of like sign. But with a very feeble charge the drops co- 

 alesce 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 elec- 

 trified, proved that the coalescence was due to a slightly different de- 

 gree of electric tension in the impinging particles of water. In such 

 case their attraction and coalescence are determined by static induc- 

 tion, 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 to- 

 gether. 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 in- 

 duction 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 apply- 

 ing mathematics to the theory of electrified spheres or drops of water. 

 An explanation of terms is essential to an understanding of the rea- 

 soning. 



