Bells are made of bronze, about three quarters copper and one quarter tin. It is thought that much copper gives a deep, full tone, and that much tin with, sometimes, zinc makes the tone sharp. The age of a bell has something to do with its sound being rich and mellow; but the bellmaker has even more, for he must understand not only how to cast it, but also how to tune it. If you tap a large bell, it will, if properly tuned, sound a clear note. Tap it just on the curve of the top, and it will give a note exactly one octave above the first. If the note of the bell is too low, it can be made higher by cutting away a little from the inner rim. If it is too high, it can be made lower by filing on the inside a little above the rim. Many of the old bells contain the gifts of silver and gold which were thrown in by people who watched their founding. The most famous bell in the United States is the "Liberty Bell" of Independence Hall, in Philadelphia, which rang when Independence was adopted by Congress. This was founded in England long before the Revolution and later was melted and founded again in the United States. It would not be easy to get on without brass and bronze; but even these alloys are not so necessary as copper by itself. It is so strong that it is used in boiler tubes of locomotives, as roofing for buildings and railroad coaches, in the great pans and vats of the sugar factories and refineries. A copper ore called "malachite," which shows many shades of green, beautifully blended and mingled, is used for the tops of tables. Wooden ships are often "copper-bottomed"; that is, sheets of copper are nailed to that part of the hull which is under water in order to prevent barnacles from making their homes on it, and so lessening the speed of the vessel. People often say that the latter half of the nineteenth century was the Age of Steel, because so many new uses for steel were found at that time. The twentieth century promises to be the Age of Electricity, and electricity must have copper. Formerly iron was used for telegraph wires; but it needs much more electricity to carry power or light or heat or a telegraphic message over an iron wire than one of copper. Moreover, iron will rust and will not stretch in storms like copper, and so needs renewing much oftener. Electric lighting and the telephone are everywhere, even on the summits of mountains and in mines a mile below the earth's surface. Electric power, if a waterfall furnishes the electricity, is the cheapest power known. The common blue vitriol is one form of copper, and to this we owe many of our electric conveniences. It is used in all wet batteries, and so it rings our doorbells for us. It also sprays our apple and peach trees, and is a very valuable article. Indeed, copper in all its forms, pure and alloyed, is one of our best and most helpful friends. IX THE NEW METAL, ALUMINUM