If you had lived in Britain a thousand years ago, you might have seen panes of glass in the churches, but you would have found very few glass windows in the houses of the people. The windows in those days were generally narrow open slits in the wall, though sometimes they were filled up with thin pieces of horn, or with thin skin.

What a difference glass windows have made in the brightness and comfort of our homes! The glass is so transparent that it lets the light from the sun pass through it, and yet it keeps out both the rain and the wind.

Glass is so hard that you cannot cut it or even scratch it with a common steel knife. The beautiful gem called the diamond is, however, even harder than glass; and glaziersuse small diamonds set in a handle to cut glass to the size that they want. Little wheels of extremely hard steel are also used for cutting glass, and this they do fairly well, though not nearly so well as diamonds.

If you hold a pane of thin glass under water, you will find that you can cut it with a pair of scissors. But you must cut near the edge of the glass, and take off only a little bit at a time. Like many other hard substances, glass is very brittle—that is, it will snap easily if we try to bend it, or if we let it fall.