53.WHAT IS AN AVALANCHE?
An avalanche is a sliding mass of snow, or ice, or wet earth and stones. An avalanche of earth and stones is a kind of landslide. It can happen even in regions without snow. A steep mountainside or bluff becomes thoroughly soaked with water, or is weakened in some other way. The earth can no longer stand in such a steep slope without sliding, and an avalanche begins.
52.WHAT IS AMBER?
In ancient times, amber was thought to have magical powers. The ancient Greeks and Romans believed it could protect people against sorcerers and disease, so they wore it. The reason amber was believed to have these mysterious powers was its ability to become electrified. When a piece of amber is rubbed briskly, a charge of electricity is produced. This makes amber attract light articles, such as bits of paper. The Greek name for amber was elektron, and this is the origin of our word “electricity.”
51.WHAT ARE CRYSTALS?
Perhaps you think of a crystal as a rare and beautiful mineral or gem. You are partly right. Emeralds and diamonds are crystals. But not an crystals are rare and beautiful. Each tiny particle of salt and sugar is also a crystal! Many of the common substances around us are crystals.
A crystal is the solid form of a substance. It has a definite shape and a definite number of faces because of the arrangement of its atoms. All crystals of the same substance have the same shape, though they may differ in size.
