Where Does Salt Come From?
Salt is one of the things with which we come in contact with daily
perhaps more than any other. With the exception of water, probably no
one thing is used more by all civilized people than salt.
You have already learned in our talk on elements the difference between
a mere mixture of substances and a chemical compound. You remember
that when some substances are only mixed together, they do not lose
their identity. In a compound the substances are always combined in
fixed proportions and the properties of the compound are often very
different from those of the things that make it. Common salt is made of
two substances, that are not at all like salt, and are very different
from each other. One, sodium, is a soft, bluish metal, and the other is
chlorine, a yellowish-green gas. The chemical name for salt is sodium
chloride which is derived from the two names sodium and chlorine.
Sodium and chlorine are both what we have learned to call elements. An
element being a substance which cannot be separated into substances
of different kinds. There are now known about seventy such elements.
All the substances around us are composed of these elements alone, or
chemically united in different compounds, or simply mixed together.
Most of them, however, are mixtures, not of separate elements, but of
compounds. The soil under our feet is a mixture of compounds. Water is
also a compound. Pure compounds very rarely occur naturally. Salt is
sometimes found almost pure; but generally is mixed with so many other
things that we have to take them out to get absolutely pure salt. For
practical every-day use it is unnecessary to purify the salt.
Salt is found in large quantities in the sea water, in which it is
dissolved with some other substances. It is also found in salt beds,
formed by the drying up of old lakes that have no outlets; salt wells,
that yield strong brine; and salt mines, in which it is found in hard,
solid, transparent crystals, called rock salt. Rock salt is the purest
form in which salt is found and, to prepare it for market, it is merely
necessary to grind it or cut into blocks. The greatest deposit of salt
in the world is probably that at Wielizka in Poland, where there is a
bed 500 miles long, 20 miles wide, and 1,200 feet thick. Some of the
mines there are so extensive that it is said some of the miners spend
all their lives in them, never coming to the surface of the earth.
A trip through these mines is interesting. In one of them can be seen
a church made entirely of salt. The salt supply of the United States
is obtained chiefly from the salt wells of Michigan and New York, the
Great Salt Lake in Utah, and the rock-salt mines of Louisiana and
Kansas.
In the arts and manufactures, the most important uses of salt are
in glazing earthenware, in extracting metals from their ores, in
preserving meats and hides, in fertilizing arid soil, and also, as we
shall presently see, in the manufacture of soda. Of equal importance,
perhaps, is its use in food. Most people think it not only lends a
pleasant flavor, but is itself an important article of diet. It is
certain, that all people who can obtain it use salt in their food, and
where it is scarce, it is considered one of the greatest of luxuries.
Soda is of interest to us, not so much on account of its use in
our households, as because it plays on extremely important part in
two industries that contribute greatly to our comfort, viz., the
manufacture of glass and soap.
Soda is not found naturally in great abundance, as salt is, but is
generally made from other substances. Formerly it was made almost
entirely from the ashes of certain plants. One, known as the Salsoda
soda-plant, was formerly cultivated in Spain for the soda contained
in it, and the ashes, or Barilla, as they were called, were soaked in
water to dissolve out the soda. Now, however, the world’s soda supply
is produced from common salt by two processes, known from the names of
their inventors as the Leblanc and Solvay processes.
~WHERE WE GET SODA~
In the Leblanc process the first step is to treat the salt, or sodium
chloride, with sulphuric acid. As a result of this, a compound of
sodium, sulphur, and oxygen, called sodium sulphate is formed, together
with another acid containing hydrogen and chlorine, and called
hydrochloric acid. This acid is driven off by boiling, and the sodium
sulphate is left.
The next step in the process is to convert the sodium sulphate, or
“salt cake,” into soda, or, to give it its chemical name, sodium
carbonate. This change is brought about by mixing the salt cake with
limestone and coal and heating the mixture. Just what changes go on
when this is done, are not known, but the chief ones are probably the
following: the coal, which consists for the most part of an element
called carbon, takes the oxygen out of the sodium sulphate, and unites
with it to form carbonic acid gas, leaving a compound of sodium and
sulphur called sodium sulphide; this acts on the limestone, which is
composed of a metal, calcium, in combination with carbon and oxygen,
and causes the sulphur in the sodium sulphide to combine with the
calcium, forming calcium sulphide, while the sodium combines with the
carbon and oxygen and forms the desired compound, sodium carbonate.
After the heating, the resulting mass which contains calcium sulphide,
sodium carbonate, and some unburned coal, and is known as “black
ash,” is broken up and treated with water. This dissolves the sodium
carbonate, leaving the rest undissolved, and when part of the water is
evaporated crystals containing sodium carbonate and water are formed.
By heating these the water may be driven off, and the sodium carbonate
left behind as a white powder.
The Solvay, or ammonia soda, process consists in forcing carbonic acid
gas through strong brine, to which a considerable quantity of ammonia
has been added. When this is done, crystals are formed in the brine,
which are composed of a compound of hydrogen, sodium, carbon, and
oxygen, and are called sodium bicarbonate. This substance, which is the
soda we sometimes use in baking bread, is decomposed by heating, into
water and sodium carbonate, the soda used for washing.
The Leblanc process was formerly used almost altogether for making
soda; but in recent years the Solvay process has come into extensive
use, and it is said that now more than half the soda of the world is
made in this way.
Where Do All the Little Round Stones Come From?
The little round stones you are thinking of are really pebbles which
have been worn smooth and round by being rubbed against each other in
the water, through the action of the waves on a beach, or the running
water of brooks and streams. This sort of rock is called a water-formed
rock. Some of them have travelled many miles before they are found
side by side on the shore or in a large mass of what we would call
conglomerate rock. But whenever you see a round smooth rock or pebble
you may be quite sure that it was made round and smooth by the action
of water.
You sometimes see large rocks made of small stones of various colors
and sizes. You can often find a large rock of this kind standing by
itself. If you examine it carefully, you will find it consists of an
immense number of small stones of different sizes and of a variety of
colors, all fastened together as though with cement. This kind of rock
is called conglomerate. We know two kinds of conglomerate rock, one,
quite common, in which the little stones are round and smooth, and
another, not seen so often, in which the stones are sharp. The latter
sort is sometimes called breccia, to distinguish it from the former,
which is called true pudding stone.
What Is Clay?
Clay is the result of the crumbling of a certain kind of rocks called
feldspars. When feldspar is exposed to the action of the weather, it
crumbles slowly at the surface and the little fragments combine with
a certain amount of water, forming clay. Pure clay is white and is
used in the manufacture of china and porcelain. The common clay that
we usually think of when we think of clay, is generally yellowish,
but there are many different colored clays. Most of these colors,
particularly those of red clay, yellow clay and blue clay, come from
the iron which is present in the clay. Clay which contains iron is
useful for making bricks. Bricks are made from clay by first softening
the clay and pressing it in molds, the size of a brick. When dried for
a time in the sun they are put into an oven and baked in great heat
and they become quite hard and generally red. Most of the clay from
which bricks are made turns red when baked, whether blue, yellow or
red, because the iron which is in the clay is generally turned red when
subjected to heat.
For making porcelains it is desirable to use the kinds of clay which
contain nothing that melts when heated to a high degree. Clays which
contain substances which melt in strong heat are, therefore, not good
for making porcelains. There is a pure white clay called Kaolin which
is very excellent for this purpose. Clay out of which we make firebrick
for lining stoves and fireplaces is free from substances which melt.
Several kinds of clay are good for making paints.
Where Do School Slates Come From?
Slates such as are used in school and as roofing material are formed of
clay, which has been hardened under pressure and heat. When this occurs
it does so because a number of layers of clay, one on top of the other,
have at sometime been subjected to great heat and pressure within the
earth with the result that the clay is pressed into very thick layers
and changed in color by the heat and becomes hard. There are many kinds
of slate. Some of the slate, as found in slate mines, is used to make
roofs over buildings and for this purpose they are cut to shapes very
much like wooden shingles. They are easily broken, however, as slate is
very brittle.
Slate is used in many other ways besides for roofs and school slates.
Sometimes it is made into slate pencils but, since paper has become
so cheap, comparatively few slate pencils are used in the school room
today.
What Causes Shadows?
Where anything through which rays of light cannot pass intercepts the
light rays coming from a luminous body, the light rays are turned back
in the direction from which they come and the part on the other side
of the object which intercepted the light goes into shade and a shadow
results. A shadow then is produced by cutting off one or more light
rays. We notice shadows when the sun is bright in the daytime and at
night when we walk along the streets lighted partly by street lamps.
The shadows we see in the daytime are caused by our cutting off and
throwing back some of the light rays which come from the sun. These are
not so dark as the shadows we see at night because the rays of light
from the sun are so bright and are reflected from so many other objects
to the side and in back of us.
When, however, we are walking along a dimly lighted street and come to
a street lamp the shadows our bodies cause are quite black. The night
shadows are darker because the source of light is less intense and the
objects to the side of and in back of us (if we are walking toward the
light) do not reflect so much of the light rays as they do of the sun’s
rays in the daytime.
[Illustration: DRIVING THE HOLLOW STEEL PILES TO BED ROCK.]