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Lesson 4 Black-lead 第四课 黑铅 What is the black-lead in your pencil made of? If you take a bit of the metal which we call lead, you will find that it is grey in colour, very heavy, and not at all like the black-lead in your pencil. But the lea
This Scientific Americans podcast is sponsored by audible.com, the number-one online resource for best-selling audio books and more. And now audible.com is offering Scientific American listeners a free audio book of your choice. For details go to aud
By Phil Mercer Sydney 29 March 2008 Australians switched off their lights for 60 minutes Saturday to mark Earth Hour, a campaign by environmentalists to raise awareness about global warming. The event was started a year ago in Sydney by the World Wil
This is the VOA Special English Health Report. Many people have low iron in their blood. But pregnant women need extra iron for their own health and their baby's health. Iron is important to the development of a baby's brain and central nervous syste
Copper can be rolled or hammered out into thin sheets, and so we say that it is malleable. It can also be easily drawn out into wire, and for this reason we call it ductile. Sheet copper is much used for covering the bottoms of ships, and for making
When the sheets are very thin, they are called lead-foil. A great deal of lead-foil is used in China, to line the chests in which tea is sent to Europe. 特别薄的薄片儿就叫做铅箔, 中国就是用大量的铅箔来垫在茶叶箱里, 做成铅