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.