WATER
65. Destructive Action of Water. The action of water in stream and
sea, in springs and wells, is evident to all; but the activity of
ground water--that is, rain water which sinks into the soil and
remains there--is little known in general. The real activity of ground
water is due to its great solvent power; every time we put sugar into
tea or soap into water we are using water as a solvent. When rain
falls, it dissolves substances floating in the atmosphere, and when it
sinks into the ground and becomes ground water, it dissolves material
out of the rock which it encounters (Fig. 30). We know that water
contains some mineral matter, because kettles in which water is boiled
acquire in a short time a crust or coating on the inside. This crust
is due to the accumulation in the kettle of mineral matter which was
in solution in the water, but which was left behind when the water
evaporated. (See Section 25.)
[Illustration: FIG. 30.--Showing how caves and holes are formed by the
solvent action of water.]
The amount of dissolved mineral matter present in some wells and
springs is surprisingly great; the famous springs of Bath, England,
contain so much mineral matter in solution, that a column 9 feet in
diameter and 140 feet high could be built out of the mineral matter
contained in the water consumed yearly by the townspeople.
[Illustration: FIG. 31.--The work of water as a solvent.]
Rocks and minerals are not all equally soluble in water; some are so
little soluble that it is years before any change becomes apparent,
and the substances are said to be insoluble, yet in reality they are
slowly dissolving. Other rocks, like limestone, are so readily soluble
in water that from the small pores and cavities eaten out by the
water, there may develop in long centuries, caves and caverns (Fig.
30). Most rock, like granite, contains several substances, some of
which are readily soluble and others of which are not readily soluble;
in such rocks a peculiar appearance is presented, due to the rapid
disappearance of the soluble substance, and the persistence of the
more resistant substance (Fig. 31).
We see that the solvent power of water is constantly causing changes,
dissolving some mineral substances, and leaving others practically
untouched; eating out crevices of various shapes and sizes, and by
gradual solution through unnumbered years enlarging these crevices
into wonderful caves, such as the Mammoth Cave of Kentucky.
66. Constructive Action of Water. Water does not always act as a
destructive agent; what it breaks down in one place it builds up in
another. It does this by means of precipitation. Water dissolves salt,
and also dissolves lead nitrate, but if a salt solution is mixed with
a lead nitrate solution, a solid white substance is formed in the
water (Fig. 32). This formation of a solid substance from the mingling
of two liquids is called precipitation; such a process occurs daily in
the rocks beneath the surface of the earth. (See Laboratory Manual.)
[Illustration: FIG. 32.--From the mingling of two liquids a solid is
sometimes formed.]
Suppose water from different sources enters a crack in a rock,
bringing different substances in solution; then the mingling of the
waters may cause precipitation, and the solid thus formed will be
deposited in the crack and fill it up. Hence, while ground water tends
to make rock porous and weak by dissolving out of it large quantities
of mineral matter, it also tends under other conditions to make it
more compact because it deposits in cracks, crevices, and pores the
mineral matter precipitated from solution.
These two forces are constantly at work; in some places the
destructive action is more prominent, in other places the constructive
action; but always the result is to change the character of the
original substance. When the mineral matter precipitated from the
solutions is deposited in cracks, _veins_ are formed (Fig. 33), which
may consist of the ore of different metals, such as gold, silver,
copper, lead, etc. Man is almost entirely dependent upon these veins
for the supply of metal needed in the various industries, because in
the original condition of the rocks, the metallic substances are so
scattered that they cannot be profitably extracted.
[Illustration: FIG. 33.--Mineral matter precipitated from solution is
deposited in crevices and forms veins.]
Naturally, the veins themselves are not composed of one substance
alone, because several different precipitates may be formed. But there
is a decided grouping of valuable metals, and these can then be
readily separated by means of electricity.
67. Streams. Streams usually carry mud and sand along with them;
this is particularly well seen after a storm when rivers and brooks
are muddy. The puddles which collect at the foot of a hill after a
storm are muddy because of the particles of soil gathered by the water
as it runs down the hill. The particles are not dissolved in the
water, but are held there in suspension, as we call it technically.
The river made muddy after a storm by suspended particles usually
becomes clear and transparent after it has traveled onward for miles,
because, as it travels, the particles drop to the bottom and are
deposited there. Hence, materials suspended in the water are borne
along and deposited at various places (Fig. 34). The amount of
deposition by large rivers is so great that in some places channels
fill up and must be dredged annually, and vessels are sometimes caught
in the deposit and have to be towed away.
Running water in the form of streams and rivers, by carrying sand
particles, stones, and rocks from high slopes and depositing them at
lower levels, wears away land at one place and builds it up at
another, and never ceases in its work of changing the nature of the
earth's surface (Fig. 35).
[Illustration: FIG. 34.--Deposit left by running water.]
[Illustration: FIG. 35.--Water by its action constantly changes the
character of the land.]
68. Relation of Water to Human Life. Water is one of the most
essential of food materials, and whether we drink much or little
water, we nevertheless get a great deal of it. The larger part of many
of our foods is composed of water; more than half of the weight of the
meat we eat is made up of water; and vegetables are often more than
nine tenths water. (See Laboratory Manual.) Asparagus and tomatoes
have over 90 per cent. of water, and most fruits are more than three
fourths water; even bread, which contains as little water as any of
our common foods, is about one third water (Fig. 36).
[Illustration: FIG. 36.--Diagram of the composition of a loaf of bread
and of a potato: 1. ash; 2, food; 3, water.]
Without water, solid food material, although present in the body,
would not be in a condition suitable for bodily use. An abundant
supply of water enables the food to be dissolved or suspended in it,
and in solution the food material is easily distributed to all parts
of the body.
Further, water assists in the removal of the daily bodily wastes, and
thus rids the system of foul and poisonous substances.
The human body itself consists largely of water; indeed, about two
thirds of our own weight is water. The constant replenishing of this
large quantity is necessary to life, and a considerable amount of the
necessary supply is furnished by foods, particularly the fruits and
vegetables.
But while the supply furnished by the daily food is considerable, it
is by no means sufficient, and should be supplemented by good drinking
water.
69. Water and its Dangers. Our drinking water comes from far and
near, and as it moves from place to place, it carries with it in
solution or suspension anything which it can find, whether it be
animal, vegetable, or mineral matter. The power of water to gather up
matter is so great that the average drinking water contains 20 to 90
grains of solid matter per gallon; that is, if a gallon of ordinary
drinking water is left to evaporate, a residue of 20 to 90 grains will
be left. (See Laboratory Manual.) As water runs down a hill slope
(Fig. 37), it carries with it the filth gathered from acres of land;
carries with it the refuse of stable, barn, and kitchen; and too often
this impure surface water joins the streams which supply our cities.
Lakes and rivers which furnish drinking water should be carefully
protected from surface draining; that is, from water which has flowed
over the land and has thus accumulated the waste of pasture and
stable and, it may be, of dumping ground.
[Illustration: FIG. 37.--As water flows over the land, it gathers
filth and disease germs.]
It is not necessary that water should be absolutely free from all
foreign substances in order to be safe for daily use in drinking; a
limited amount of mineral matter is not injurious and may sometimes be
really beneficial. It is the presence of animal and vegetable matter
that causes real danger, and it is known that typhoid fever is due
largely to such impurities present in the drinking water.
70. Methods of Purification. Water is improved by any of the
following methods:--
(_a_) _Boiling_. The heat of boiling destroys animal and vegetable
germs. Hence water that has been boiled a few minutes is safe to use.
This is the most practical method of purification in the home, and is
very efficient. The boiled water should be kept in clean, corked
bottles; otherwise foreign substances from the atmosphere reënter the
water, and the advantage gained from boiling is lost.
(_b_) _Distillation_. By this method pure water is obtained, but this
method of purification cannot be used conveniently in the home
(Section 25).
(_c_) _Filtration_. In filtration, the water is forced through
porcelain or other porous substances which allow the passage of water,
but which hold back the minute foreign particles suspended in the
water. (See Laboratory Manual.) The filters used in ordinary dwellings
are of stone, asbestos, or charcoal. They are often valueless, because
they soon become choked and cannot be properly cleaned.
The filtration plants owned and operated by large cities are usually
safe; there is careful supervision of the filters, and frequent and
effective cleanings are made. In many cities the filtration system is
so good that private care of the water supply is unnecessary.
71. The Source of Water. In the beginning, the earth was stored with
water just as it was with metal, rock, etc. Some of the water
gradually took the form of rivers, lakes, streams, and wells, as now,
and it is this original supply of water which furnishes us all that we
have to-day. We quarry to obtain stone and marble for building, and we
fashion the earth's treasures into forms of our own, but we cannot
create these things. We bore into the ground and drill wells in order
to obtain water from hidden sources; we utilize rapidly flowing
streams to drive the wheels of commerce, but the total amount of water
remains practically unchanged.
The water which flows on the earth is constantly changing its form;
the heat of the sun causes it to evaporate, or to become vapor, and to
mingle with the atmosphere. In time, the vapor cools, condenses, and
falls as snow or rain; the water which is thus returned to the earth
feeds our rivers, lakes, springs, and wells, and these in turn supply
water to man. When water falls upon a field, it soaks into the ground,
or collects in puddles which slowly evaporate, or it runs off and
drains into small streams or into rivers. That which soaks into the
ground is the most valuable because it remains on the earth longest
and is the purest.
[Illustration: FIG. 38.--How springs are formed. _A_, porous layer;
_B_, non-porous layer; _C_, spring.]
Water which soaks into the ground moves slowly downward and after a
longer or shorter journey, meets with a non-porous layer of rock
through which it cannot pass, and which effectually hinders its
downward passage. In such regions, there is an accumulation of water,
and a well dug there would have an abundant supply of water. The
non-porous layer is rarely level, and hence the water whose vertical
path is obstructed does not "back up" on the soil, but flows down hill
parallel with the obstructing non-porous layer, and in some distant
region makes an outlet for itself, forming a spring (Fig. 38). The
streams originating in the springs flow through the land and
eventually join larger streams or rivers; from the surface of streams
and rivers evaporation occurs, the water once more becomes vapor and
passes into the atmosphere, where it is condensed and again falls to
the earth.
Water which has filtered through many feet of earth is far purer and
safer than that which fell directly into the rivers, or which ran off
from the land and joined the surface streams without passing through
the soil.
72. The Composition of Water. Water was long thought to be a simple
substance, but toward the end of the eighteenth century it was found
to consist of two quite different substances, oxygen (O) and hydrogen
(H.)
[Illustration: FIG. 39.--The decomposition of water.]
If we send an electric current through water (acidulated to make it a
good conductor), as shown in Figure 39, we see bubbles of gas rising
from the end of the wire by which the current enters the water, and
other bubbles of gas rising from the end of the wire by which the
current leaves the water. These gases have evidently come from the
water and are the substances of which it is composed, because the
water begins to disappear as the gases are formed. If we place over
each end of the wire an inverted jar filled with water, the gases are
easily collected. The first thing we notice is that there is always
twice as much of one gas as of the other; that is, water is composed
of two substances, one of which is always present in twice as large
quantities as the other.
73. The Composition of Water. On testing the gases into which water
is broken up by an electric current, we find them to be quite
different. One proves to be oxygen, a substance with which we are
already familiar. The other gas, hydrogen, is new to us and is
interesting as being the lightest known substance, being even "lighter
than a feather."
An important fact about hydrogen is that in burning it gives as much
heat as five times its weight of coal. Its flame is blue and almost
invisible by daylight, but intensely hot. If fine platinum wire is
placed in an ordinary gas flame, it does not melt, but if placed in a
flame of burning hydrogen, it melts very quickly.
74. How to prepare Hydrogen. There are many different methods of
preparing hydrogen, but the easiest laboratory method is to pour
sulphuric acid, or hydrochloric acid, on zinc shavings and to collect
in a bottle the gas which is given off. This gas proves to be
colorless, tasteless, and odorless. (See Laboratory Manual.)