Can we form any idea why the crystals build themselves up so
systematically? Dr. Tyndall says we can, and I hope by the help
of these small bar magnets to show you how he explains it. These
little pieces of steel, which I hope you can see lying
on this white cardboard, have been rubbed along a magnet until
they have become magnets themselves, and I can attract and lift
up a needle with any one of them. But if I try to lift one bar
with another, I can only do it by bringing certain ends together.
I have tied a piece of red cotton (c, Fig. 21) round one end of
each of the magnets, and if I bring two red ends together they
will not cling together but roll apart. If, on the contrary, I
put a red end against an end where there is not cotton, then the
two bars cling together. This is because every magnet has two
poles or points which are exactly opposite in character, and to
distinguish them one is called the positive pole and the other
the negative pole. Now when I bring two red ends, that is, two
positive poles together, they drive each other away. See! the
magnet I am not holding runs away from the other. But if I bring
a red end and a black end, that is, a positive and a negative end
together, then they are attracted and cling. I will make a
triangle (A, Fig. 21) in which a black end and a red end always
come together, and you see the triangle holds together. But now if
I take off the lower bar and turn it (B, Fig. 21) so that two red
ends and two black ends come together, then this bar actually
rolls back from the others down the cardboard. If I were to break
these bars into a thousand pieces, each piece would still have two
poles, and if they were scattered about near each other in such a
way that they were quite free to move, they would arrange
themselves always so two different poles came together.
Now picture to yourselves that all the particles of those
substances which form crystals have poles like our magnets, then
you can imagine that when the heat which held them apart is
withdrawn and the particles come very near together, they will
arrange themselves according to the attraction of their poles and
so build up regular and beautiful patterns.
So, if we could travel up to the clouds where this fairy power of
crystallization is at work, we should find the particles of
water-vapour in a freezing atmosphere being built up into minute
solid crystals of snow. If you go out after a snow-shower and
search carefully, you will see that the snow-flakes are not mere
lumps of frozen water, but beautiful six-pointed crystal stars, so
white and pure that when we want to speak of anything being
spotlessly white, you say that it is "white as snow." Some of
these crystals are simply flat slabs with six sides, others are
stars with six rods or spikes springing from the centre, others
with six spikes each formed like a delicate fern. No less than a
thousand different forms of delicate crystals have been found
among snowflakes, but though there is such a great variety, yet
they are all built on the six-sided and six-pointed plan, and are
all rendered dazzlingly white by the reflection of the light from
the faces of the crystals and the tiny air-bubbles built up within
them. This, you see, is why, when the snow melts, you have only a
little dirty water in your hand; the crystals are gone and there
are no more air-bubbles held prisoners to act as looking-glasses
to the light. Hoar-frost is also made up of tiny water-crystals,
and is nothing more than frozen dew hanging on the blades of grass
and from the trees.
But how about ice? Here, you will say, is frozen water, and yet
we see no crystals, only a clear transparent mass. Here, again,
Dr. Tyndall helps us. He says (and as I have proved it true, so
may you for yourselves, if you will) that if you take a
magnifying glass, and look down on the surface of ice on a sunny
day, you will see a number of dark, six-sided stars, looking like
flattened flowers, and in the centre of each a bright spot. These
flowers, which are seen when the ice is melting, are our old
friends the crystal stars turning into water, and the
bright spot in the middle is a bubble of empty space, left
because the watery flower does not fill up as much room as the
ice of the crystal star did.
And this leads us to notice that ice always takes up more room
than water, and that this is the reason why our water-pipes burst
in severe frosts; for as the water freezes it expands with great
force, and the pipe is cracked, and then when the thaw comes on ,
and the water melts again, it pours through the crack it has
made.
It is not difficult to understand why ice should take more room;
for we know that if we were to try to arrange bricks end to end
in star-like shapes, we must leave some spaces between, and could
not pack them so closely as if they lay side by side. And so,
when this giant force of crystallization constrains the atoms of
frozen water to grow into star-like forms, the solid mass must
fill more room than the liquid water, and when the star
melts, this space reveals itself to us in the bright spot of the
centre.
We have now seen our drop of water under all its various forms of
invisible gas, visible steam, cloud, dew, hoar-frost, snow, and
ice, and we have only time shortly to see it on its travels, not
merely up and down, as hitherto, but round the world.
We must first go to the sea as the distillery, or the place from
which water is drawn up invisibly, in its purest state, into the
air; and we must go chiefly to the seas of the tropics, because
here the sun shines most directly all the year round, sending
heat-waves to shake the water-particles asunder. It has been
found by experiment that, in order to turn 1 lb. of water into
vapour, as much heat must be used as is required to melt 5 lbs.
of iron; and if you consider for a moment how difficult iron is
to melt, and how we can keep an iron poker in a hot fire and yet
it remains solid, this will help you to realize how much heat the
sun must pour down in order to carry off such a constant supply
of vapour from the tropical seas.
Now, when all this vapour is drawn up into the air, we know that
some of it will form into clouds as it gets chilled high up in
the sky, and then it will pour down again in those tremendous
floods of rain which occur in the tropics.
But the sun and air will not let it all fall down at once, and
the winds which are blowing from the equator to the poles carry
large masses of it away with them. Then, as you know, it will
depend on many things how far this vapour is carried. Some of it,
chilled by cold blasts, or by striking on cold mountain tops, as
it travels northwards, will fall in rain in Europe and Asia, while
that which travels southwards may fall in South America,
Australia, or New Zealand, or be carried over the sea to the South
Pole. Wherever it falls on the land as rain, and is not used by
plants, it will do one of two things; either it will run down in
streams and form brooks and rivers, and so at last find its way
back to the sea, or it will sink deep in the earth till it comes
upon some hard rock through which it cannot get, and then, being
hard pressed by the water coming on behind, it will rise up again
through cracks, and come to the surface as a spring. These
springs, again, feed rivers, sometimes above- ground, sometimes
for long distances under-ground; but one way or another at last
the whole drains back into the sea.
But if the vapour travels on till it reaches high mountains in
cooler lands, such as the Alps of Switzerland; or is carried to
the poles and to such countries as Greenland or the Antarctic
Continent, then it will come down as snow, forming immense snow-
fields. And here a curious change takes place in it. If you make
an ordinary snowball and work it firmly together, it becomes very
hard, and if you then press it forcibly into a mould you can turn
it into transparent ice. And in the same way the snow which falls
in Greenland and on the high mountains of Switzerland becomes
very firmly pressed together, as it slides down into the valleys.
It is like a crowd of people passing from a broad thoroughfare
into a narrow street. As the valley grows narrower and
narrower the great mass of snow in front cannot move down
quickly, while more and more is piled up by the snowfall behind,
and the crowd and crush grow denser and denser. In this way the
snow is pressed together till the air that was hidden in its
crystals, and which gave it its beautiful whiteness, is all
pressed out, and the snow-crystals themselves are squeezed into
one solid mass of pure, transparent ice.
Then we have what is called a "glacier," or river of ice, and
this solid river comes creeping down till, in Greenland, it
reaches the edge of the sea. There it is pushed over the brink of
the land, and large pieces snap off, and we have "icebergs."
These icebergs - made, remember, of the same water which was
first draw up from the tropics - float on the wide sea, and
melting in its warm currents, topple over and over* (A floating
iceberg must have about eight times as much ice under the water
as it has above, and therefore, when the lower part melts in a
warm current, the iceberg loses its balance and tilts over, so as
to rearrange itself round the centre of gravity.) till they
disappear and mix with the water, to be carried back again to the
warm ocean from which they first started. In Switzerland the
glaciers cannot reach the sea, but they move down into the
valleys till they come to a warmer region, and there the end of
the glacier melts, and flows away in a stream. The Rhone and many
other rivers are fed by the glaciers of the Alps; and as these
rivers flow into the sea, our drop of water again finds its way
back to its home.
But when it joins itself in this way to its companions, from whom
it was parted for a time, does it come back clear and transparent
as it left them? From the iceberg it does indeed return pure and
clear; for the fairy Crystallization will have no impurities, not
even salt, in her ice-crystals, and so as they melt they give back
nothing but pure water to the sea. Yet even icebergs bring down
earth and stones frozen into the bottom of the ice, and so they
feed the sea with mud.
But the drops of water in rivers are by no means as pure as when
they rose up into the sky. We shall see in the next lecture how
rivers carry down not only sand and mud all along their course,
but even solid matter such as salt, lime, iron, and flint,
dissolved in the clear water, just as sugar is dissolved, without
our being able to see it. The water, too, which has sunk down
into the earth, takes up much matter as it travels along. You all
know that the water you drink from a spring is very different
from rain-water, and you will often find a hard crust at the
bottom of kettles and in boilers, which is formed of the
carbonate of lime which is driven out of the clear water when it
is boiled. The water has become "hard" in consequence of having
picked up and dissolved the carbonate of lime on its way through
the earth, just in the same way as water would become sweet if
you poured it through a sugar-cask. You will also have heard of
iron-springs, sulphur-springs, and salt-springs, which come out
of the earth, even if you have never tasted any of them, and the
water of all these springs finds its way back at last to the
sea.
And now, can you understand why sea-water should taste
salt and bitter? Every drop of water which flows from the earth
to the sea carries something with it. Generally, there is so
little of any substance in the water that we cannot taste it, and
we call it pure water; but the purest of spring or river-water
has always some solid matter dissolved in it, and all this goes
to the sea. Now, when the sun-waves come to take the water out of
the sea again, they will have nothing but the pure water itself;
and so all these salts and carbonates and other solid substances
are left behind, and we taste them in sea-water.
Some day, when you are at the seaside, take some extra water and
set it on the hob till a great deal has simmered gently away, and
the liquid is very thick. Then take a drop of this liquid, and
examine it under a microscope. As it dries up gradually, you will
see a number of crystals forming, some square - and these will be
crystals of ordinary salt; some oblong - these will be crystals
of gypsum or alabaster; and others of various shapes. Then, when
you see how much matter from the land is contained in sea-water,
you will no longer wonder that the sea is salt; on the contrary,
you will ask, Why does it not grow salter every year?
The answer to this scarcely belongs to our history of a drop of
water, but I must just suggest it to you. In the sea are numbers
of soft-bodied animals, like the jelly animals which form the
coral, which require hard material for their shells or the solid
branches on which they live, and they are greedily watching for
these atoms of lime, of flint, or magnesia, and of other
substances brought down into the sea. It is with lime and magnesia
that the tiny chalk-builders form their beautiful shells, and the
coral animals their skeletons, while another class of builders use
the flint; and when these creatures die, their remains go to form
fresh land at the bottom of the sea; and so, though the earth is
being washed away by the rivers and springs it is being built up
again, out of the same materials, in the depths of the great
ocean.
And now we have reached the end of the travels of our drop of
water. We have seen it drawn up by the fairy "heat," invisible
into the sky; there fairy "cohesion" seized it and formed it into
water-drops and the giant, "gravitation," pulled it down again to
the earth. Or, if it rose to freezing regions, the fairy of
"crystallization" built it up into snow-crystals, again to fall
to the earth, and either to be melted back into water by heat, or
to slide down the valleys by force of gravitation, till it became
squeezed into ice. We have detected it, when invisible, forming a
veil round our earth, and keeping off the intense heat of the
sun's rays by day, or shutting it in by night. We have seen it
chilled by the blades of grass, forming sparkling dew-drops or
crystals of hoar-frost, glistening in the early morning sun; and
we have seen it in the dark underground, being drunk up greedily
by the roots of plants. We have started with it from the tropics,
and travelled over land and sea, watching it forming rivers, or
flowing underground in springs, or moving onwards to the high
mountains or the poles, and coming back again in glaciers and
icebergs. Through all this, while it is being carried
hither and thither by invisible power, we find no trace of its
becoming worn out, or likely to rest from its labours. Ever
onwards it goes, up and down, and round and round the world,
taking many forms, and performing many wonderful feats. We have
seen some of the work that it does, in refreshing the air,
feeding the plants, giving us clear, sparkling water to drink,
and carrying matter to the sea; but besides this, it does a
wonderful work in altering all the face of our earth. This work
we shall consider in the next lecture, on "The two great
Sculptors - Water and Ice."