Why is it that of the whole surface of this globe, we may consider that
three-fourths are covered by water, and that only one-fourth is in a
condition to be permanently inhabited by human beings? Is there any great
object in nature served by this? Is there any law of nature which would
prevent the proportion being one-fourth water to three-fourths land, or
even less water? In fact, what after all is the great use of water upon
the large scale in nature?
First of all, although three-fourths of the globe are now covered with
water, there is no reason to suppose that it has been always so. On the
contrary, it is quite certain that the proportion between land and water
has changed very much and very frequently; that the whole continent of
Europe was at one time the bed of an immense sea, when probably there was
a great continent where the Pacific Ocean is now spread; that even Old
Ireland was once not merely what Admiral Yorke wished her to be,
forty-eight hours under water, but probably many thousand years in that
condition; and that the great tract of limestone which occupies all the
centre of the country, is nothing more than a collection of the skeletons
of shell-fish, her first inhabitants, which by time and pressure have been
converted into the hard material of which we build our houses, and which
we burn into lime. There is thus no particular reason why there should be
three times as much water at present as land, but it is easy to show that
water on the great, as well as on the small scale, is of paramount
importance in nature.
Water is a portion of the food of all living beings. In the case of
animals, the bodies from whence they derive nutriment are so varied and so
complex, that to illustrate the peculiar part which water plays in each,
would occupy too much space. In all our drinks, even in ardent spirits,
there is a very large quantity of water, and our solid food very seldom
contains less than nine-tenths of its weight of water. The living body is
even less solid. A man weighing 150 lbs. would, if perfectly dried, weigh
not more than 10 lbs., the other 140 lbs. being water. It is to the
existence of this quantity of water that we owe the elasticity, the
softness, and pliability of the different portions of our frame, the
animal tissues being, when dry, hard and brittle as dry glue.
The nutrition of vegetables furnishes a beautiful and simple example of
the use of water in nature. The body of the vegetable, the proper wood,
may be considered as being composed of water and of charcoal; and hence,
when we heat a piece of wood until we decompose it, the water is expelled,
and carbon or charcoal remains behind. In order to grow, a plant must
therefore get water and charcoal in a form fit for its use, that is, in
such a form as it can make food of, and digest them. For this, the carbon
is supplied in the carbonic acid which the air contains, and the water in
the state of vapour which the air contains also, and which is continually
descending under the form of dew and rain to moisten the leaves and the
roots of the plants, when it has been absorbed into the ground. All the
water which is absorbed by plants is not assimilated, or digested; a great
part is again thrown out by the surface of the leaves; for, precisely as
the air which an animal expires from the lungs in breathing is loaded with
vapour, so is there a process of perspiration from the surface of the
leaves, which are the lungs of plants. For the formation of substances
which are peculiar to certain plants, other substances are required as
food, thus, most plants require nitrogen, which is accordingly furnished
abundantly in atmospheric air; others must have access to sulphur, in
order to flourish; but this depends, as it were, upon particular branches
of manufacture in which the plant is engaged; for its own support, for
making wood, and the tissue of its leaves and vessels, it uses only water
and carbonic acid.
The conversion of water into steam or invisible vapour by boiling, is one
of the best known facts in science; but by a little attention we can
observe that this change takes place at almost all temperatures, although
much less rapidly. Thus, if a little water be laid in a plate, it is soon
dried up, and wet clothes, by being hung up in the air, are very soon
completely dried. Even below the temperature at which water freezes, it
still evaporates; and thus, when a fall of snow is succeeded by a
continued frost, the snow gradually disappears from the fields without
having melted, evaporating while yet solid. From the surface of all the
water of the globe, therefore, there is continually ascending a stream of
watery vapour; but as the proportion of sea is so much greater than that
of land, we may look upon the ocean as being the source of the watery
vapour of the air upon the large scale.
Now, watery vapour is lighter than air, and hence the vapour, as soon as
formed, ascends in the air like a balloon, until it arrives at a part of
the air which is of its own specific gravity. The air in these higher
regions is extremely cold, and the vapour can no longer maintain itself
under the form of invisible steam: it is condensed, and would immediately
fall back to its source as rain or hail, but for a singular property which
it acquires at the moment of being vaporised. When water evaporates, it
becomes highly electrified, and could attract a feather, or other light
bodies, like a stick of sealing-wax which has been rubbed briskly on a
woollen cloth. Now, the vapour which passes off is electrified also; and
while in this state of electricity, it, on arriving at the colder regions
of the air, cannot condense, to form liquid water. The minute particles of
the water repel each other too violently, in virtue of their
electricities, to form drops, but they constitute the great loose
collections of clouds which diversify so much the appearance of our sky.
The clouds being thus highly electrical, and being very light, are
attracted by the tops of mountains and high lands, or by elevated
buildings; and, giving off their electricity, the particles of water
coalesce, to form drops which descend as rain. In this country the air is
so damp that in general the discharge of the electricity of the clouds
takes place quietly and silently; but in summer, and in dry climates, it
produces the vivid flashings and injurious effects of the lightning, and
the re-echoed rattle of the thunder-clap.
When water is cooled, it diminishes in bulk like other bodies; but at a
particular temperature it deviates from the general law of contraction,
and by doing so, becomes, perhaps, the most striking example of
providential design that is to be met with in inorganic nature. Cold water
is specifically heavier than warm water, in consequence of the contraction
it has undergone, and hence will sink in it, as water would sink in oil.
Now, if we consider the surface of a lake exposed to the cooling action of
a wintry wind, the water which is first cooled becomes heavier, and,
sinking to the bottom, is replaced by the warmer water, which floats up to
the top; there is thus a current established of cold water descending and
of warmer water rising up. This continues until all the water in the lake
has been cooled down to the temperature at which its specific gravity is
greatest, which is about 40 degrees, or about eight degrees above the
point at which it begins to freeze. The action of the cold wind
continuing, the water at the surface is still further cooled; but now, in
place of contracting, it expands--instead of becoming heavier, it becomes
lighter, and remains floating upon the surface. It is then still further
cooled, and finally its temperature being reduced to 32 degrees, it
freezes, and a layer of ice is formed on the surface of the lake. This
ice, and the cold water next it, are impermeable to heat: it actually
serves as a blanket to the water at 40 degrees which is below, preventing
the escape of the heat, and retaining it at that temperature, sufficient
for the purposes to which it is subservient; for at the temperature of 40
degrees, the life and enjoyments of all the various tribes of animals and
vegetables which reside permanently under the surface of the water are
perfectly secured, at least for a very considerable time; the water
holding dissolved a quantity of oxygen for the animal respiration, and the
vegetables living on the carbonic acid which is formed by the respiration
of the fish. On the approach of spring, the warmer air, and the rays of
the more elevated sun, act directly on the surface of the ice, and each
portion of water formed by melting, becoming heavier, sinks, so as to
expose the ice itself to the source of heat. Thus the ice is rapidly
dissolved, and after a few days the lake throws off its wintry aspect
altogether.
Now, if water did not possess this peculiarity of being heaviest at the
particular temperature of 40 degrees--if it contracted according as it was
cooled, up to the moment of freezing, as almost all other liquids do, what
would be the result? The cold wind acting on the surface of the lake, and
the water becoming heavier by being cooled, the circulation would continue
until all the water had been cooled to the point at which it freezes. The
ice would then form indifferently in all portions of it, at the bottom and
in the centre, as well as on the surface; and by the continued action of
the source of cold, the wind, the whole mass of water in the lake would be
frozen into a solid block of ice. The watery sap in the vessels of the
aquatic plants, the blood in fishes and other animals inhabiting the
water, would be equally frozen, and all these living beings consequently
killed. Further, on the approach of summer, by the first heating action
of the air and sun, a layer of ice, of a few inches thick upon the
surface, would be melted, but the water thus produced would, by being
impenetrable to heat, prevent the great body of ice below from being
affected. Just as, in reality, the cold water at the surface prevents the
warmer water below from being cooled, so then it would prevent the colder
ice below from being warmed; and hence the heats of summer passing over
without the melting process extending beyond a few feet in depth, the
first cold days of the next winter would solidify all again.
In every country, therefore, where at present water is frozen at all in
winter, we should have there established the reign of perpetual frost. By
the presence of such large masses of ice, the temperature of the ground
would be so much reduced, that, in place of the rich herbage of our
meadows, and the luxuriant produce of our corn-fields, we should have our
country yielding a scanty support to wandering herds of deer, in the
mosses and lichens that could be scraped up from beneath the snow. The
oaks, the beeches, the horse-chesnuts, which give such beauty to our
sylvan scenery, would disappear, and the monotony of wildernesses of the
Scotch fir and of the spruce would be varied only by patches of stunted
birch. The countries nearer the tropics would be gradually brought into
the same condition, by the depression of their mean temperature; and thus,
in a short time after water had ceased to possess this peculiar property,
the whole surface of the globe would be reduced to the condition of which
we now happily only read in the tales of the arctic voyagers; and all
commerce, manufactures, and civilization, would be banished from the
earth. Of such value is this little peculiarity of water!
A property of water, which, however, unlike the former, it shares with all
other liquids, is, that when it freezes it gives out a large quantity of
heat; and that conversely, in order that ice may melt, it must obtain,
from some other source, a quantity equally considerable. Consequently,
water freezes and ice melts very slowly; and that it should melt thus
slowly, is of essential importance in animated nature. If in spring or
summer, when vegetable life is in activity, when the development of
leaves, of flowers, and fruit, is at its greatest energy, end all the
vessels of the plant are distended with its nutritious juices, were it
suddenly exposed to cold, the sap would be frozen, and by the expansion of
the ice the vegetable tissues torn to pieces, and the plant killed. In the
thin extremities, as in the leaves, such is the effect of the frost of a
single night; but as the fluids, yielding but gradually up their latent
heat, solidify very slowly, the injury does not extend so far as to be
beyond the remedial powers of the plant itself. In another way, however,
the peculiar latent heat of water is of still more importance. If there
was no large collection of water on the globe, the change of seasons would
be amazingly more rapid and more remarkable than they at present are. A
change in the direction of the wind, the alteration which a few weeks
should effect in the position of the sun, would transfer us from the depth
of the severest colds of winter to the summer heats. These colds and heats
would also be much greater than they at present are, and an approximation
to this actually occurs in countries far distant from the sea. The central
districts of Europe and of Asia have what are termed continental climates
to distinguish them from ours, which is called insular. Their summers are
hotter, their winters are much colder, and the spring and autumn seasons
of passage, which with us might be said to occupy most of the year, are in
those countries of only a few weeks’, or even a few days’, duration. In
fact, when on the cessation of summer the first cold winds tend to bring
on the winter, and to bind up our lakes in frost, the first portion of
water frozen becomes, by giving up its latent heat, a source of warmth
which tempers the chilly air, and retards its action on the remainder. The
water freezes thus very slowly. The vegetables, and certain classes of
animals, feeling the cold of winter thus gradually coming on, prepare to
meet it without injury. The motion of the sap in the one, that of the
blood in the other class of living beings, becomes slower, and, dropping
its leaves and fruit, the tree retains but its firm trunk, within which
its energies are preserved for the ensuing season; whilst the hedgehog,
the viper, the frog, and other animals, retire to their hiding-places, and
in a state of almost lifeless stupor remain until the warmth of the
succeeding spring calls them to renewed existence.
In the formation of the insular climate which we possess, another power of
water, however, equally or perhaps more influential, can be traced. There
issues continually from the ocean at the equator, as the earth revolves, a
current of water considerably warmer than that which bathes our shores.
This current becoming sensible first in the Gulf of Mexico, is called the
Gulf Stream; it passes obliquely across the Atlantic, floating on the
colder water of the ocean, which tends in a direction nearly opposite to
replace it, and thus diffuses over the coasts of North America and Europe
the heat which it had absorbed within the torrid zone. The northerly
winds, which would bring down a sudden winter on us, are therefore
tempered by passing over the warmer surface of the ocean; whilst the hot
winds from the south, which on the approach of spring might make too
premature a change, expend, in passing over the great expanse of sea, a
portion of their heat; and thus the transition in both directions is
rendered more gradual and harmless.
These are but a few of the important duties which are allotted to water in
its place in nature. It in other respects presents an equally interesting
subject of examination, and it is one to which we shall return. From its
value as the great agent of nutrition to the vegetable world, and the
necessity of a supply of it to animals; from its power in modifying the
appearance and structure of a country, changing land into sea, and
elevating banks where deep water had been before, the philosophers of old
looked upon water as the origin of all earthly things, as being above all
others the element of nature. It is not so: water is not an element. Among
other wonders which chemistry has taught us, we have learned of what water
is composed; and on another occasion we shall describe the way in which
its elements may be obtained.