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HUMIDITY IN THE ATMOSPHERE
FTER a good rain the fields are muddy; thousands and
thousands of threads of water running down the slopes
have filled the ditches or collected in puddles and
pools; the foliage of the trees shows a glossy green
and shines as if the watery coating left by the storm
were so much varnish; at the end of each leaf, at the
tip of the tiniest blade of grass, trembles a drop of
water, flashing like a diamond in the sunlight.
"But wait a few days and if the sun is hot and the wind
blows a little, there will not be left a trace of this
shower that gladdened the farmer's heart. The earth
will have become dusty again, in the woods the little
cushions of moss that but lately were so exquisitely
fresh will have shriveled up and faded, the leaves of
the trees will hang limp and lifeless, the puddles will
be dry, and the mud will have turned to dust.
"What has become of the quantities of water discharged
by the storm? The soil drank part of it, that is
certain, to the benefit of the growing vegetation; but
the air—insatiable toper!—also took its
share, and a large one. The water that fell in the form
of rain from the upper air has by evaporation returned
to the atmosphere whence it came; the air
 has taken back what it gave to the soil for a little
while. The drops that sparkled at the ends of the
leaves have returned, invisible, to the immensities of
the atmosphere without touching the earth. In short,
evaporation has dissipated the waters of the storm into
"Countless bodies of water, running or stationary, are
constantly giving off moisture from their surface by
evaporation. Little brooks hastening to unite and form
a rivulet, rivulets contributing their waters to make a
river, rivers emptying into larger rivers, and these in
turn discharging their floods into the sea; lakes,
ponds, marshes, stagnant pools—all, absolutely
all, even to the smallest puddle no larger than the
hollow of one's hand, yield vapor to the atmosphere
without a moment's pause.
"To picture to oneself the total volume of water thus
rising continually into the air would be beyond the
power of the imagination; and yet that is a mere
nothing, for we are forgetting the chief source of
atmospheric humidity. We forget the sea, the immense
sea, covering three quarters of the earth's
surface—the prodigious sea, in comparison with
which all the rivers combined are as a mere trifle.
What is a drop of water compared with a mill-pond?
Nothing. It is the same with the waters irrigating the
continents when we contrast their combined volume with
the vastness of the sea.
"From the oceans and the bodies of water on the various
continents there rises unceasingly into the atmosphere
an inconceivable quantity of vapor. Now, this does not
stay long over the bodies of water
 that gave it forth; the wind carries it away, to-day in
one direction, to-morrow in another, sometimes to
immense distances, so that a layer of air saturated
with moisture a thousand leagues from here may reach us
and furnish the air we breathe.
"Through the agency of storms, hurricanes, and all
kinds of winds, gentle or boisterous, that agitate the
air, there is brought about an indiscriminate
redistribution, in every direction, of the water vapor
that rose from any given part of sea or land. In this
way the air about us holds moisture, always and
"Yes, this air that surrounds us now, this air in which
we come and go, contains water in the form of invisible
vapor; and it always contains it, sometimes more,
sometimes less, every moment and in all seasons. You
will be convinced of this if I succeed in making
visible to you what is now invisible; if, in short, I
succeed in reconverting into water, running water, this
vapor which no eye can see.
"As it required only a slight rise in temperature to
convert ordinary water into invisible vapor, it will
suffice to lower the temperature a little, and thus
cool the vapor, to restore it to its former condition,
or in other words to reconvert it into water. This
cooling process, this taking away of heat, will undo
what warming or the application of heat and done in the
first place. That is all plain enough, it seems to me.
"Let us go back a moment to the pot of boiling water.
Cover it with the lid, first wiping the latter
perfectly dry inside; or, better still, hold the lid
 the steam escaping from the pot, and at a short
distance from the mouth.
"You foresee what will happen. The inside of the lid,
perfectly dry at first, will in a few moments be
covered with drops of water. Where do these drops come
from if not from the steam which, by contact with the
cold lid, has lost its heat, to which it owed its
subtile form, and has returned to its original state,
that of water? So much, then, we have demonstrated,
that cold changes vapor to water; which is exactly the
opposite of the change wrought by heat.
"For the sake of brevity we will in our future talks
use a few terms which I will here define. The return of
vapor to the state of water is called condensation. The
opposite term is evaporation, which designates the
conversion of water into vapor. If we wish to say that
vapor becomes water again, we say it condenses.
"How shall we make the invisible vapor that is in the
air manifest itself as water? Nothing easier, if we
have some ice or snow at our disposal. We fill a
water-bottle with small pieces of ice, wipe the outside
well to remove any moisture that may already be there,
and put the bottle on a perfectly dry plate.
"We do not have to wait long for the result. The glass
of the water-bottle, at first perfectly clear, becomes
dull and veiled in a kind of fog. Then little drops
form, grow bigger, and slowly run down into the plate.
Wait a quarter of an hour and we shall have enough
water in the plate to pour into a glass and drink if we
"Where does this water come from, if you please?
 Certainly not from the inside of the water-bottle, for
water cannot pass through glass. Then it must come from
the surrounding air, which by contact with the
ice-cooled glass has itself become so chilled as to
make its vaporous contents first appear as a king of
fog, the initial step in condensation, and then run
down the glass in the form of drops. In this manner it
may be proved at any season of the year that there is a
certain amount of moisture in the air.
"We do not always have ice or show at our disposal,
especially in summer. During that season must we, then,
forego this interesting and instructive experiment?
Certainly not. We merely have to fill the water-bottle
with very cold water and we shall see the glass become
dull and clouded with moisture.
"There may even be some formation of drops large enough
to run down the sides. The result will be the same as
when ice is used, only less pronounced, because the
cooling effect of cold water is less than that of ice.
"This phenomenon may have been presented to our view
many times without attracting our notice. The carafe of
cold water placed on our table at dinner very soon
loses its transparency and turns dull from the
condensation of vapor on the outside. A glass filled
with cold water ceases to be transparent, becomes
covered with a dull cloud, and looks as if badly
washed. That, too, is the vapor from the surrounding
air condensing—and collecting on the cold object."