【英语时差8,16】火是什么？

It's one of our most important tools, and holds a prominent place in many ancient philosophies and religions.   The ancient Greeks believed that fire–along with earth, water, and air–was one of the four essential elements that made up the world. We now know that the world's a lot more complicated, with over a hundred elements of matter which can be combined in a tremendous variety of ways. This might leave you wondering where fire fits in. What exactly is fire?

Watching a flame dance through the air, you might conclude that fire's a gas, like oxygen or carbon dioxide. It's not. Fire can burn fuel that's a gas, or a liquid, or even a solid–as in the case of glowing charcoal¹. But the fire itself isn't any of these things. In fact, fire isn't any thing at all. It's not its own type of matter; it's something that matter can do. Fire is a chemical reaction.

A fire needs oxygen and some kind of fuel. This fuel–whether it's candle wax, wood, or gasoline–usually contains big molecules² that have carbon atoms inside them. You can think of these molecules as little containers of energy. When they're allowed to combine with oxygen, this energy is released as heat and light.

Fire is a rapid chemical reaction known as oxidation. Inside a fire, oxygen molecules break bigger molecules apart into carbon dioxide and water vapor³. All the heat and light of a fire comes from big, carbon-based molecules combining with oxygen.

So what is fire? It's not the fuel or the oxygen or the heat or the light. Fire is what happens between all these things. It's a chemical reaction.  

1 charcoal   

n.炭，木炭，生物炭

参考例句：

• We need to get some more charcoal for the barbecue.我们烧烤需要更多的碳。
• Charcoal is used to filter water.木炭是用来过滤水的。

2 molecules   

分子( molecule的名词复数 )

参考例句：

• The structure of molecules can be seen under an electron microscope. 分子的结构可在电子显微镜下观察到。
• Inside the reactor the large molecules are cracked into smaller molecules. 在反应堆里，大分子裂变为小分子。

3 vapor   

n.蒸汽，雾气

参考例句：

• The cold wind condenses vapor into rain.冷风使水蒸气凝结成雨。
• This new machine sometimes transpires a lot of hot vapor.这部机器有时排出大量的热气。