Assumption of a Solar Electric Potential. 179 
In this greater humidity of the higher strata of the atmo- 
sphere is to be sought the reason why even with descending 
currents of air falls of rain may occur. If the temperature 
of a very moist upper current be cooled by radiation below 
the point of saturation of the vapour, cirrus clouds will be 
formed, probably consisting of ice crystals *. The latent heat 
of the vapour and water thereby set free will again heat these 
strata of the air and protract the process of formation of hea- 
vier snow-clouds ; but if by continued loss of heat by radiation 
the process be completed, the weight of the icej no longer 
filling any considerable space, must disturb the adiabatic equi- 
librium, and a sinking of the mass of cloud commence. The 
condensation and heating then taking place melt the snow 
again ; and the requisite latent heat is withdrawn from the 
air. The adiabatic equilibrium is hence still further disturbed; 
and the final result will be a cold descending current with 
rain. The density of these slowly descending rain-clouds, 
however, will not be great enough to make the cloud elec- 
trically conductive ; consequently there will be no formatiot. 
of electricity by distribution. The course of things, however, 
will be different when by local overheating of the air in the 
vicinity of the ground a local upcurrent with rainfall is pro- 
duced. The upcurrent may then acquire a velocity greater 
than that of the falling drops formed in the resisting air ; hence 
these will be whirled with the current into the upper regions, 
the temperature of which lies far below the freezing-point, 
and frozen to hailstones. By the rapid increase of volume, and 
the corresponding lateral expansion of the accelerated current, 
the next higher strata of air, which are relatively moist and cold, 
are set whirling with a horizontal axis of rotation ; and these 
whirlwinds combine with that which is ascending and rotating 
about a vertical axis. The violent whirling motion into which 
* It is extremely probable, however, that in the high, regions of the 
atmosphere both water and vapour retain their state of aggregation down 
to far below the temperature of their points of freezing and condensation 
respectively. That water without the presence of solid bodies to induce 
crystalhzatiou, aud without violent agitation, can be cooled far below 
—20° C. without freezing is a fact. That steam in like manner can 
retain the form of vapour below its point of condensation has not yet 
been proved experimentally. We know only the retardation of boil- 
ing, which so frequently gives rise to steam-boiler explosions. It is at 
all events not improbable that this retardation of boiling also stands 
opposed to a retardation of condensation. This can only with difficulty 
be ascertained by experiments, since means are wanting to cool a mass of 
steam out of contact with solid or liquid bodies. Without assuming this 
it cannot well be explained why the sky is not always entirely covered 
with cirrus clouds ; for it would have to be assumed that water particles 
liquefied in the great rarefaction of the higher strata of air do not appear 
as clouds. 
P2 
