Glass

As early as 500 BC, metal workers noticed that solidified slag had properties not unlike those of the highly prized volcanic glass obsidian. This made people want to heat up all sorts of rocks in various ways. Slags of many colors were produced as well as transparent ones. When set to a useful purpose, such as in the making of beads, containers, or windows, the hardened slag became known as **glass**. Over many centuries, glass making gradually improved.

Clearer and stronger glass made many important inventions possible. Eyeglasses were first invented in Italy in the thirteenth century, and their use quickly spread, because it made people intellectually active throughout their lives. In 1610, the telescope was pointed skyward by Galileo. He observed moons orbiting Jupitar and opened the door to the astronomy age. Glass was later improved to design distillation equipment used to isolate alcohol from fermented broths. This liquid was called distilled spirits. Distilled spirits was noted for their intoxicating effects but also for their ability to disinfect and promote the healing of wounds.

Pieces of glass can be melted together to become sealed without glue or cement. Glass is transparent and resistant to the most corrosive chemicals, which is why chemical reactions in labs are usually carried out in glass containers. Long, thin strands of glass are flexible enough to be in cables that can stretch for hundreds of kilometers. These strands are called **fiber-optic cables**. These cables carry information data with great efficiency by pulses of light.

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