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;