The next thing we have to account for is the bed of shale or
hardened clay covering over the coal. Now we know that from time
to time land has gone slowly up and down on our globe so as in
some places to carry the dry ground under the sea, and in others
to raise the sea-bed above the water. Let us suppose, then, that
the great Dismal Swamp was gradually to sink down so that the sea
washed over it and killed the reeds and shrubs. Then the streams
from the west would not be sifted any longer but would bring down
mud, and leave it, as in the delta of the Nile or Mississippi, to
make a layer over the dead plants. You will easily understand
that this mud would have many pieces of dead trees and plants in
it, which were stifled and died as it covered them over; and thus
the remains would be preserved like those which we find now in
the roof of the coal-galleries.
But still there are the thick sandstones in the coal-mine to be
explained. How did they come there? To explain them, we must
suppose that the ground went on sinking till the sea covered the
whole place where once the swamp had been, and then sea-sand
would be thrown down over the clay and gradually pressed down by
the weight of new sand above, till it formed solid sandstone and
our coal-bed became buried deeper and deeper in the earth.
At last, after long ages, when the thick mass of sandstones above
the bed b (Fig. 46) had been laid down, the sinking must have
stopped and the land have risen a little, so that the sea was
driven back; and then the rivers would bring down earth again and
make another clay-bed. Then a new forest would spring up, the
ferns, Calamites, Lepidodendrons, and Sigillarias would gradually
form another jungle, and many hundred of feet above the buried
coal-bed b, a second bed of peat and vegetable matter would begin
to accumulate to form the coal-bed a.
Such is the history of how the coal which we now dig out of the
depths of the earth once grew as beautiful plants on the surface.
We cannot tell exactly all the ground over which these forests
grew in England, because some of the coal they made has been
carried away since by rivers and cut down by the waves of the
sea, but we can say that wherever there is coal now, there they
must have been then.
Try and picture to yourselves that on the east coast of
Northumberland and Durham, where all is now black with coal-
dust, and grimy with the smoke of furnaces; and where the noise
of hammers and steam-engines, and of carts and trucks hurrying to
and fro, makes the country re-echo with the sound of labour;
there ages ago in the silent swamp shaded with monster trees, one
thin layer of plants after another was formed, year after year,
to become the coal we now value so much. In Lancashire, busy
Lancashire, the same thing was happening, and even in the middle
of Yorkshire and Derbyshire the sea must have come up and washed
a silent shore where a vast forest spread out over at least 700
or 800 square miles. In Stafford-shire, too, which is now almost
the middle of England, another small coal-field tells the same
story, while in South Wales the deep coal-mines and number of
coal-seams remind us how for centuries and centuries forests must
have flourished and have disappeared over and over again under
the sand of the sea.
But what is it that has changed these beds of dead plants into
hard, stony coal? In the first place you must remember they have
been pressed down under an enormous weight of rocks above them.
We can learn something about this even from our common lead
pencils. At one time the graphite or pure carbon, of which the
blacklead (as we wrongly call it) of our pencils is made, was dug
solid out of the earth. but so much has now been used that they
are obliged to collect the graphite dust, and press it under a
heavy weight, and this makes such solid pieces that they can cut
them into leads for ordinary cedar pencils.
Now the pressure which we can exert by machinery is absolutely
nothing compared to the weight of all those hundreds of feet of
solid rock which lie over the coal-beds, and which has pressed
them down for thousands and perhaps millions of years; and
besides this, we know that parts of the inside of the earth are
very hot, and many of the rocks in which coal is found are
altered by heat. So we can picture to ourselves that the coal
was not only squeezed into a solid mass, but often much of the
oil and gas which were in the leaves of the plants was driven out
by heat, and the whole baked, as it were, into one substance.
The difference between coal which flames and coal which burns
only with a red heat, is chiefly that one has been baked and
crushed more than the other. Coal which flames has still got in
it the tar and the gas and the oils which the plant stored up in
its leaves, and these when they escape again give back the
sunbeams in a bright flame. The hard stone coal, on the contrary,
has lost a great part of these oils, and only carbon remains,
which seizes hold of the oxygen of the air and burns without
flame. Coke is pure carbon, which we make artificially by driving
out the oils and gases from coal, and the gas we burn is part of
what is driven out.
We can easily make coal-gas here in this room. I have brought a
tobacco-pipe, the bowl of which is filled with a little powdered
coal, and the broad end cemented up with Stourbridge clay. When
we place this bowl over a spirit-lamp and make it very hot, the
gas is driven out at the narrow end of the pipe and lights easily
(see Fig. 53). This is the way all our gas is made, only that
furnaces are used to bake the coal in, and the gas is passed into
large reservoirs till it is wanted for use.
You will find it difficult at first to understand how coal can be
so full of oil and tar and gases, until you have tried to think
over how much of all these there is in plants, and especially in
seeds - think of the oils of almonds, of lavender, of cloves, and
of caraways; and the oils of turpentine which we get from the
pines, and out of which tar is made. When you remember these and
many more, and also how the seeds of the club-moss now are
largely charged with oil, you will easily imagine that the large
masses of coal-plants which have been pressed together and broken
and crushed, would give out a great deal of oil which, when made
very hot, rises up as gas. You may often yourself see tar oozing
out of the lumps of coal in a fire, and making little black
bubbles which burst and burn. It is from this tar that James
Young first made the paraffin oil we burn in our lamps, and the
spirit benzoline comes from the same source.
From benzoline, again, we get a liquid called aniline, from which
are made so many of our beautiful dyes - mauve, magenta, and
violet; and what is still more curious, the bitter almonds, pear-
drops, and many other sweets which children like to well, are
actually flavoured by essences which come out of coal-tar. Thus
from coal we get not only nearly all our heat and our light, but
beautiful colours and pleasant flavours. We spoke just now of
the plants of the coal as being without beautiful flowers, and
yet we see that long, long after their death they give us lovely
colours and tints as beautiful as any in flower-world now.
Think, then, how much we owe to these plants which lived and died
so long ago! If they had been able to reason, perhaps they might
have said that they did not seem of much use in the world. They
had no pretty flowers, and there was no one to admire their
beautiful green foliage except a few croaking reptiles, and
little crickets and grasshoppers; and they lived and died all on
one spot, generation after generation, without seeming to do much
good to anything or anybody. Then they were covered up and put
out of sight, and down in the dark earth they were pressed all
out of shape and lost their beauty and became only black, hard
coal. There they lay for centuries and centuries, and thousands
and thousands of years, and still no one seemed to want them.
At last, one day, long, long after man had been living on the
earth, and had been burning wood for fires, and so gradually
using up the trees in the forests, it was discovered that this
black stone would burn, and from that time coal has been becoming
every day more and more useful. Without it not only should we
have been without warmth in our houses, or light in our streets
when the stock of forest-wood was used up; but we could never
have melted large quantities of iron-stone and extracted the
iron. We have proof of this in Sussex. The whole country is
full of iron-stone, and the railings of St. Paul's churchyard are
made of Sussex iron. Iron-foundries were at work there as long
as there was wood enough to supply them, but gradually the works
fell into disuse, and the last furnace was put out in the year
1809. So now, because there is no coal in Sussex, the iron lies
idle, while in the North, where the iron-stone is near the coal-
mines, hundreds of tons are melted out every day.
Again, without coal we could have had no engines of any kind, and
consequently no large manufactories of cotton goods, linen goods,
or cutlery. In fact, almost everything we use could only have
been made with difficulty and in small quantities; and even if we
could have made them it would have been impossible to have sent
them so quickly all over the world without coal, for we could
have had no railways or steamships, but must have carried all
goods along canals, and by slow sailing vessels. We ourselves
must have taken days to perform journeys now made in a few hours,
and months to reach our colonies.
In consequence of this we should have remained a very poor
people. Without manufactories and industries we should have had
to live chiefly by tilling the ground, and everyone being obliged
to toil for daily bread, there would have been much less time or
opportunity for anyone to study science, or literature, or
history, or to provide themselves with comforts and refinements
of life.
All this then, those plants and trees of the far-off ages, which
seemed to lead such useless lives, have done and are doing for
us. There are many people in the world who complain that life is
dull, that they do not see the use of it, and that there seems no
work specially for them to do. I would advise such people,
whether they are grown up or little children, to read the story
of the plants which form the coal. These saw no results during
their own short existences, they only lived and enjoyed the
bright sunshine, and did their work, and were content. And now
thousands, probably millions, of years after they lived and died,
England owes her greatness, and we much of our happiness and
comfort, to the sunbeams which those plants wove into their
lives.
They burst forth again in our fires, in our brilliant lights, and
in our engines, and do the greater part of our work; teaching us
"That nothing walks with aimless feet
That not one life shall be destroyed,
Or cast as rubbish to the void,
When God hath made the pile complete."