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an obstacle to the passage of aerial electricity. We have 
thus a wall of condensed atmosphere on either side of the 
path taken by the electricity, while the path itself is a 
vacuum. The instantaneous condensation of probably many 
hundreds of millions of cubic feet of air, weighing hundreds 
of thousands of tons, evolves so much heat as to render 
the whole luminous, and we consequently have the flash of 
lightning composed merely of walls of light surround- 
ing and corresponding in form with the path of the elec- 
tricity. 
Following the electricity down the vacuum is an enormous 
quantity of rain, the drops of which have acquired a rotatory 
motion around their own axes in addition to the downward 
one due to gravity and the electric attraction of the earth. 
Suppose them to descend with their axes of revolution hori- 
zontal. Then those in the centre removed from the influence 
of the surrounding heat will rapidly freeze and descend to 
the earth as solid balls of ice. Those with a pole pointing 
towards the source of heat, i.e., the air which condensation has 
rendered luminous, will be modified in character by that, and 
while the portion pointing inwards will be solidified, that 
pointing outwards will be merely encrusted, the influence of 
evaporation being partly counteracted by radiant heat and 
partly by the less perfect character of the vacuum. Those 
which chanced to descend with their equatorial portions brought 
successively under the influence of the heat, by having their 
axes of revolution parallel or nearly parallel to the condensed 
wall of atmosphere, will merely be encrusted equally on all 
sides, while if they chance to be so near that evaporation is 
compensated by radiation of heat from the luminous air, they 
will descend as rain, and, if nearer to the heat, they may even 
become vaporized and again ascend with the heated air to 
form portions of fresh clouds. 
The observed phenomena indicate that the vacuum is filled 
