英语时差：碳，氧，氢（下）

That’s because when carbon and oxygen merge to form carbon dioxide, for example, they give off energy. If the merging process is very slow–say in a decaying woodpile–that energy will be very subtle. Speed it up a bit, though, and the energy can be felt as heat. Even more and you can see it as light. And that’s what fire is: light and heat coming from the rapid combination of atoms. Now, all living things are chock full of carbon and hydrogen. And the air has plenty of oxygen in it. So when a pile of dead wood is left out for a while, the carbon and hydrogen atoms slowly combine with oxygen, leaving behind a gradually diminished pile. There are also microorganisms helping things along. But even without them, there would still be the slow, smokeless burning of decay.