激光的特性、原理和用途
What do a DVD player, supermarket scanner, printer, fiber optic internet connection, and industrial metal cutter have in common? They […]
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色彩即元素
A pale red and yellow rug,a blue shirt,a green jade figurine —what different color do you see when you look around?Most of the colors in our clothing and other items made by people come from dyes and pigments.Many parts of the natural world have stunning colors too.Flowers and animals come in a rainbow of shades.Gemstones are colorful too:jade is often green,rubies are usually red,amber is typically yellow,and amethyst is purple..
有机化学 101
A carbon atom possesses four electrons in its outermost orbit. To reach stability, it therefore tends to form four covalent bonds. By sharing these four electrons, it acquires an additional four electrons from other atoms, completing its outermost shell with a total of eight electrons. Atoms that readily share electrons with carbon, thus forming bonds, include hydrogen, oxygen, nitrogen, halogens, and, naturally, other carbon atoms. These bonds are referred to as carbon-carbon bonds, carbon-hydrogen bonds, carbon-oxygen bonds, etc. (Note they are all covalent bonds.)
关于衰老，动物教会了我们什么
Aging is a fact of life. Even if you avoid bad accidents, injuries, and infections, you won’t stay healthy forever. Your body will break down as you get older. You’ll get wrinkles and achy joints. Your vision and hearing will fade. Your chance of developing cancer, heart disease, and many other ailments will greatly increase. Aging “bedevils us all,” writes Steven N. Austad in his book, Methuselah’s Zoo. He defines aging as “the progressive deterioration over time of bodily functions and defenses along with increasing susceptibility to diseases.”
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时间的开始与结束
No stars . No planets . Not even any atoms . At the moment of its birth, the universe was emptiness. Then, in a burst of inflation—a bit like a balloon blowing up—a very tiny area grew much, much larger. This happened when the age of the universe was just “a trillionth of a trillionth of a trillionth of a second”, says Alexei Filippenko. He’s an astronomer at the University of California, Berkeley. The dramatic period is extremely brief. During inflation, the universe’s size doubled around at least 100 times.
一款软件是如何开发出来的
When you think of software development, you probably think first of a person writing code. In fact, for any important piece of software, there will be many people involved and only some are developers.
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生态城市
Wild nature seems to be the opposite of a human city. In nature, curvy and messy lines dominate, while in a city, most lines are straight and neat. In nature, the weather changes from rain to heat to cold, while in a city, roofs and heating and cooling systems keep people comfortable. In nature, growth happens chaotically, while in a city, growth is planned and controlled. In a city, people often pave over nature or fence it in. They tame it to the point that it may be hard to notice. But nature is always there. In fact, it is essential.
城市的地下
When you think of a city, you probably think of busy streets and towering skyscrapers . But the city doesn’t just reach up into the sky. It also extends down below the earth. Hidden beneath buildings and roads, a complex network of wires and pipes brings residents electricity, water, and gas and also removes their waste. Underground tunnels house subway systems, storage rooms, or other structures. In many cities, space above ground is so precious and crowded that engineers are finding new, creative ways to use underground space. Singapore is one of those cities. Here, the underground is already used for many purposes, and the city government is looking into ways to build even more underground structures. “You can build up, but there is a limit,” said John Keung when he was CEO of Singapore’s Building and Construction Authority. “The only thing left is to go underground.”