A DROP OF WATER. gg 
the bottom of the ice, and so they feed the sea with 
mud. 
Yet 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 that rivers not only carry down 
sand and mud all along their course, but also contain 
solid matter such as salt, lime, iron, and flint, dis- 
solved 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 dis- 
solved 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 some- 
thing with it. Generally, there is so little of any sub- 
stance 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, 
