1. The children had much to say that evening about Joe and the field. They had brought home a lump of chalk.

2. 'I will show you something,' said father, and he got a cup of vinegar, crushed a little of the chalk, and dropped it into the cup.

Fizz and bubble, bubble and fizz!

3. What was going on?

When the stir came to an end, the chalk was not there!

'Part of it has gone off in gas,' their father said. 'The rest is lime, and it is mixed with the vinegar.'

4. 'We did not see any gas,' said Harry.

'You can't see gas. It is like air. All those bubbles were made by the gas. It went out of the cup into the air.

'Now, get a cup of water. Come along! Where is your chalk?'

5. Father rubbed some of it into the water, and stirred it up. The water now looked like milk.

Father poured it into the sink, and showed Harry and Dora, at the bottom of the cup, a great many tiny grains.

6. 'Those little round things,' he said, 'are shells.'

'Shells!' said Dora, trying to see them better.

'Were live things ever in them?' asked Harry, and put a finger into the cup to fish some out.

7. 'Yes, long, long ago. That bit of chalk had hundreds and hundreds of shells in it. Now, mother, it is your turn! I have had mine. What do you know about chalk?'

8. Mother smiled and began: 'There was once a very deep sea, full of live things, little and big. And on the top of the water were crowds of tiny things in shells, that put out long arms like threads to catch their food.

9. 'When they died they all dropped to the bottom of the sea, and lay there. The shells were so very little that they made a sort of mud when they were mixed with the water.

'And now the mud is dry, and we call it chalk!'