A DROP OF WATER. 95 
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 
