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 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 no 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 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 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which form crystals have poles like our magnets, then you can imagine that when the heat which held them apart is withdrawn and the particles come very near together, they will arrange themselves according to the attraction of their poles and so build up regular and beautiful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-vapor 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 snow-flakes, but though there is such a great 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 vapor, 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 vapor from the tropical seas.

Now, when all this vapor 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 vapor 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 underground; but one way or another at last the whole drains back into the sea.

But if the vapor 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 drawn up from the tropics—float on the wide sea, and melting in its warm currents, topple over and over 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 sea-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, of 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 traveled 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 labors. 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."