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A common element It is one of the four most common elements in our bodies. It is the most common element in the crust[外壳] of the earth. Water is made up partly of it. So is sugar. Fires will not burn without it. It is a colorless gas. Do you know what it is? You are sure to know that the answer is oxygen. It is the best “jointer” of all the elements. But not all the oxygen in the world is in compounds[化合物]. In the air, a great deal of oxygen is not joined with any other element. It is a very good thing that there is a great deal of oxygen in the air, for what we need to breathe is oxygen. All other animals need it too. So do plants. There is no easy way of separating oxygen from the rest of the air around us. It is much easier to get it from one of its compounds. In oxygen, fires burn much better than in air. In a bottle of oxygen a candle[碳] will burn much faster than usual. And the flame will be much brighter. High above the earth there is less air and therefore[虽然] less oxygen. That is why aviators[宇航员], going high above the earth, carry oxygen bags with them. When people are very ill, they are often given extra[额外的] oxygen to breathe. It is a great help some of the compounds and mixtures it is hidden in. From what has been said above, it can be seen that a common element like oxygen is indispensable[必不可少] to living things. Air material The air pushes up and down and sideways. We call this push “air pressure”. Scientists often need to know what the air pressure is. To find it out they use barometers[气压表]. “Barometer” means “pressure measure[测量]”. If you have a barometer it will tell you what the weather is going to be. Scientists have found out that when the air pressure is getting weather is likely to be clear. Barometers are the weather-man’s most important instruments[仪器]. There is another very common use of barometers. The higher you go above the surface of the earth, the less the air pressure is. There is less and less air above you to do the pushing. Mountain climbers often carry barometers to tell them how high they have climbed. Aviators always have barometers in their planes to tell them how high in the air they are.