ises
into the air, carries with it its portion, however minute that portion may
be, of the earth's electric charge. This small charge distributes itself
over the surface of the aqueous particle, and the vapor rises higher and
higher until it reaches that point above which the air is too rare to
support it. It then flows away laterally, and as it approaches colder
regions gets denser, sinking lower and nearer to the earth's surface. The
aqueous particles becoming reduced in size, the extent of their surfaces
is proportionately reduced. It follows that as the particles and their
surfaces are reduced, the charge is confined to a smaller surface, and
attains, therefore, a greater "surface density," or in simpler language, a
greater amount of electricity per unit of surface.

Electricity, as above set forth, is in what is known as the "static"
condition (to distinguish it from electricity which is being transferred
in the form of a current), when it has the property of "repelling itself"
to the utmost limits of any conductor upon which it may be confined. This
will account for the charge finding its way to the surface of the water
particles, and will furthermore account for the greater density of the
charge as the particle gets smaller and has the extent of its surface
rapidly diminished. It may be mentioned that the surface of a sphere
varies as the cube of its radius.

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 s