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objects

this

double

process

ordinarily

works but

unobtrusiveeffects.

Only in a

few cases,

among

which that

of a cloud

is the

most

familiar,does

the

conflict

produce

rapid and

marked

transformations.

One of

thesefloating

bodies of

vapour

expands

and

dissipates,

if the

amount of

molecularmotion

it

receives

from the

Sun and

Earth

exceeds

that which

it loses

byradiation

into space

and

towards

adjacent

surfaces;

while,

contrariwise,if,

drifting

over cold

mountain-tops,