Returning to the discussion of the state of affairs existing when the particles have reached their highest position in the atmosphere, we may imagine that they set themselves off on journeys toward either the north or the south pole.  As they pass from the hotter to the colder regions, a number of particles coalesce; these again combine with others on the road until the vapor becomes visible as cloud.  The increased density implies increased weight, and the cloud particles, as they sail poleward, descend toward the surface of the earth.  Assuming that a spherical form is maintained throughout, the condensation of a number of particles implies a considerable reduction of surface.  Thus, the contents of two spheres vary as the cubes of their radii, or eight (the cube of 2) drops on combining will form a drop twice the radius of one of the original drops.  We may safely conceive hundreds and thousands of such combinations to take place until a cloud mass is formed, in which the constituent parts are more or less in contact, and, therefore, behave electrically as a single conductor of irregular surface, upon which is accumulated all the electricity that was previously distributed over the surfaces of the millions of particles that now compose it.

The tendency of an electric charge upon the surface of a conductor is to take upon itself a position in which it may approach nearest to an equal and opposite charge; or, if possible, to attain neutrality.  If, then, a cloud has a charge, and there is no other cloud above or near it, the charge induces on the adjacent earth surface electricity of the opposite kind.  Thus, assuming the cloud to be charged with positive electricity, the subjacent earth will be in the negative state.  The two electricities[3] exert a strong tendency to combine or to produce neutrality, whence there is a species of stress applied to the intervening air.  Possibly the cloud will be drawn bodily toward the earth more or less rapidly, according as the charge is great or small.  Or, on the other hand, the cloud may roll on for leagues, carrying its influence with it, so that the various portions of the earth underneath become successively charged and discharged as the cloud progresses on its journey.

[Footnote 3:  We may speak of two electricities or two electric states without necessarily implying adherence either to the single or the double “fluid” theory.  Whether electricity be of two kinds or no, the fact remains that there are two conditions, and all the features of this paper may be explained with equal facility by the supporters of either hypothesis.]

Should the cloud be near the earth, or should it be very highly charged, the tension of the two electricities may be so great as to overcome the resistance of the intervening air; and if this resistance should prove too weak, what happens?  How does the discharge show itself?  It takes place in the form of a lightning flash, and passing from the one surface to the other—­or, maybe, simultaneously from both—­produces neutrality more or less complete.