And now, suppose that while these or any other kind of clouds are
overhead, there comes along either a very cold wind, or a wind
full of vapour. As it passes through the clouds, it makes them
very full of water, for, if it chills them, it makes the water-
dust draw more closely together; or, if it brings a new load of
water-dust, the air is fuller than it can hold. In either case a
number of water-particles are set free, and our fairy force
"cohesion" seizes upon them at once and forms them into large
water-drops. Then they are much heavier than the air, and so they
can float no longer, but down they come to the earth in a shower
of rain.
There are other ways in which the air may be chilled, and rain
made to fall, as, for example, when a wind laden with moisture
strikes against the cold tops of mountains. Thus the Khasia Hills
in India which face the Bay of Bengal, chill the air which
crosses them on its way from the Indian Ocean. The wet winds are
driven up the sides of the hills, the air expands, and the vapour
is chilled, and forming into drops, falls in torrents of rain.
Sir J. Hooker tells us that as much as 500 inches of rain fell in
these hills in nine months. That is to say, if you could measure
off all the ground over which the rain fell, and spread the whole
nine months' rain over it, it would make a lake 500 inches, or
more than 40 feet deep! You will not be surprised that the
country on the other side of these hills gets hardly any rain,
for all the water has been taken out of the air before
it comes there. Again for example in England, the wind comes to
Cumberland and Westmorland over the Atlantic, full of vapour, and
as it strikes against the Pennine Hills it shakes off its watery
load; so that the lake district is the most rainy in England,
with the exception perhaps of Wales, where the high mountains
have the same effect.
In this way, from different causes, the water of which the sun
has robbed our rivers and seas, comes back to us, after it has
travelled to various parts of the world, floating on the bosom of
the air. But it does not always fall straight back into the
rivers and seas again, a large part of it falls on the land, and
has to trickle down slopes and into the earth, in order to get
back to its natural home, and it is often caught on its way
before it can reach the great waters.
Go to any piece of ground which is left wild and untouched you
will find it covered with grass weeds, and other plants; if you
dig up a small plot you will find innumerable tiny roots creeping
through the ground in every direction. Each of these roots has a
sponge-like mouth by which the plant takes up water. Now, imagine
rain-drops falling on this plot of ground and sinking into the
earth. On every side they will find rootlets thirsting to drink
them in, and they will be sucked up as if by tiny sponges, and
drawn into the plants, and up the stems to the leaves. Here, as
we shall see in Lecture VII., they are worked up into food for
the plant, and only if the leaf has more water than it needs,
some drops may escape at the tiny openings under the
leaf, and be drawn up again by the sun-waves as invisible vapour
into the air.
Again, much of the rain falls on hard rock and stone, where it
cannot sink in, and then it lies in pools till it is shaken apart
again into vapour and carried off in the air. Nor is it idle
here, even before it is carried up to make clouds. We have to
thank this invisible vapour in the air for protecting us from the
burning heat of the sun by day and intolerable frost by night.
Let us for a moment imagine that we can see all that we know
exists between us and the sun. First, we have the fine ether
across which the sunbeams travel, beating down upon our earth
with immense force, so that in the sandy desert they are like a
burning fire. Then we have the coarser atmosphere of oxygen and
nitrogen atoms hanging in this ether, and bending the minute sun-
waves out of their direct path. But they do very little to hinder
them on their way, and this is why in very dry countries the
sun's heat is so intense. The rays beat down mercilessly, and
nothing opposes them. Lastly, in damp countries we have the
larger but still invisible particles of vapour hanging about
among the air-atoms. Now, these watery particles, although they
are very few (only about one twenty-fifth part of the whole
atmosphere), do hinder the sun-waves. For they are very greedy of
heat, and though the light-waves pass easily through them, they
catch the heat-waves and use them to help themselves to expand.
And so, when there is invisible vapour in the air, the sunbeams
come to us deprived of some of their heat-waves, and we
can remain in the sunshine without suffering from the heat.
This is how the water-vapour shields us by day, but by night it
is still more useful. During the day our earth and the air near
it have been storing up the heat which has been poured down on
them, and at night, when the sun goes down, all this heat begins
to escape again. Now, if there were no vapour in the air, this
heat would rush back into space so rapidly that the ground would
become cold and frozen even on a summer's night, and all but the
most hardy plants would die. But the vapour which formed a veil
against the sun in the day, now forms a still more powerful veil
against the escape of the heat by night. It shuts in the heat-
waves, and only allows them to make their way slowly upwards from
the earth - thus producing for us the soft, balmy nights of
summer and preventing all life being destroyed in the winter.
Perhaps you would scarcely imagine at first that it is this screen
of vapour which determines whether or not we shall have dew upon
the ground. Have you ever thought why dew forms, or what power has
been at work scattering the sparkling drops upon the grass?
Picture to yourself that it has been a very hot summer's day, and
the ground and the grass have been well warmed, and that the sun
goes down in a clear sky without any clouds. At once the heat-
waves which have been stored up in the ground, bound back into the
air, and here some are greedily absorbed by the vapour, while
others make their way slowly upwards. The grass, especially, gives
out these heat-waves very quickly, because the blades, being very
thin, are almost all surface. In consequence of this they part
with their heat more quickly than they can draw it up from the
ground, and become cold. Now the air lying just above the grass is
full of invisible vapour, and the cold of the blades, as it
touches them, chills the water- particles, and they are no longer
able to hold apart, but are drawn together into drops on the
surface of the leaves.
We can easily make artificial dew for ourselves. I have here a
bottle of ice which has been kept outside the window. When I
bring it into the warm room a mist forms rapidly outside the
bottle. This mist is composed of water-drops, drawn out of the
air of the room, because the cold glass chilled the air all round
it, so that it gave up its invisible water to form dew-drops.
Just in this same way the cold blades of grass chill the air
lying above them, and steal its vapour.
But try the experiment, some night when a heavy dew is expected,
of spreading a thin piece of muslin over some part of the grass,
supporting it at the four corners with pieces of stick so that it
forms an awning. Though there may be plenty of dew on the grass
all round, yet under this awning you will find scarcely any. The
reason of this is that the muslin checks the heat-waves as they
rise from the grass, and so the grass-blades are not chilled
enough to draw together the water-drops on their surface. If you
walk out early in the summer mornings and look at the fine cobwebs
flung across the hedges, you will see plenty of drops on the
cobwebs themselves sparkling like diamonds; but underneath on the
leaves there will be none, for even the delicate cobweb has been
strong enough to shut in the heat-waves and keep the leaves warm.
Again, if you walk off the grass on to the gravel path, you find
no dew there. Why is this? Because the stones of the gravel can
draw up heat from the earth below as fast as they give it out,
and so they are never cold enough to chill the air which touches
them. On a cloudy night also you will often find little or no dew
even on the grass. The reason of this is that the clouds give
back heat to the earth, and so the grass does not become chilled
enough to draw the water-drops together on its surface. But after
a hot, dry day, when the plants are thirsty and there is little
hope of rain to refresh them, then they are able in the evening
to draw the little drops from the air and drink them in before
the rising sun comes again to carry them away.
But our rain-drop undergoes other changes more strange than
these. Till now we have been imagining it to travel only where
the temperature is moderate enough for it to remain in a liquid
state as water. But suppose that when it is drawn up into the air
it meets with such a cold blast as to bring it to the freezing
point. If it falls into this blast when it is already a drop,
then it will freeze into a hailstone, and often on a hot summer's
day we may have a severe hailstorm, because the rain-drops have
crossed a bitterly cold wind as they were falling, and have been
frozen into round drops of ice.
But if the water-vapour reaches the freezing air while it is still
an invisible gas, and before it has been drawn into a drop, then
its history is very different. The ordinary force of cohesion has
then no power over the particles to make them into watery globes,
but its place is taken by the fairy process of "crystallization,"
and they are formed into beautiful white flakes, to fall in a
snow-shower. I want you to picture this process to yourselves, for
if once you can take an interest in the wonderful power of nature
to build up crystals, you will be astonished how often you will
meet with instances of it, and what pleasure it will add to your
life.
The particles of nearly all substances, when left free and not
hurried, can build themselves into crystal forms. If you melt
salt in water and then let all the water evaporate slowly, you
will get salt-crystals; -- beautiful cubes of transparent salt
all built on the same pattern. The same is true of sugar; and if
you will look at the spikes of an ordinary stick of sugar-candy,
such as I have here, you will see the kind of crystals which
sugar forms. You may even pick out such shapes as these
from the common crystallized brown sugar in the sugar basin, or
see them with a magnifying glass on a lump of white sugar.
But it is not only easily melted substances such as sugar and
salt which form crystals. The beautiful stalactite grottos are
all made of crystals of lime. Diamonds are crystals of carbon,
made inside the earth. Rock-crystals, which you know probably
under the name of Irish diamonds, are crystallized quartz; and
so, with slightly different colourings, are agates, opals,
jasper, onyx, cairngorms, and many other precious stones. Iron,
copper, gold, and sulphur, when melted and cooled slowly build
themselves into crystals, each of their own peculiar form, and we
see that there is here a wonderful order, such as we should never
have dreamt of, if we had not proved it. If you possess a
microscope you may watch the growth of crystals yourself by
melting some common powdered nitre in a little water till you
find that no more will melt in it. Then put a few drops of this
water on a warm glass slide and place it under the microscope. As
the drops dry you will see the long transparent needles of nitre
forming on the glass, and notice how regularly these crystals
grow, not by taking food inside like living beings, but by adding
particle to particle on the outside evenly and regularly.