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第1段
1 .Listen to a conversation between a student and his chemistry professor.
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2 .(professor) How are you, Ned?
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3 .(male student) Very well. Thanks. Um, last month we were talking, and you mentioned doing ceramics? So, like I told you, I do ceramics, too. And my exhibitor’s finally been scheduled. It\'s this weekend at the gallery by the State Park. So, if you have time...
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4 .(professor) Thank you, Ned. I\'d be delighted to come.
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5 .(male student) Great. Also, I was hoping you could answer a chemistry related question. I noticed that the clay I use in the studio has an unusual fragrance. They say it\'s local clay and this might sound strange. But it reminds me of the scent in the air right after a rainfall. Is this just my imagination?
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6 .(professor) Not at all. Actually, what you\'re smelling is called petrichor.
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7 .(male student) Petrichor?
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8 .(professor) Right. Petrichor is a mixture of plant oils and a chemical compound made by bacteria. What happens is that plants produce certain oils during dry periods. And these oils mix with the soil, especially if it has lots of clay.
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9 .And as long as the ground stays dry, the oils keep seeds from sprouting, and make it difficult for nearby plants to grow and compete for limited water supplies, until it rains again.
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10 .(male student) And then?
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11 .(professor) Well, first, you mentioned the State Park, right? In forested areas, bacteria in the soil make another chemical compound during dry weather that mixes with the plant oils.
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12 .And when it rains, the force of the raindrops hitting the ground, sort of squirt this fragrant mixture, this petrichor, up into the air.
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13 .(male student) So, that good smell actually comes mostly from the ground?
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14 .(professor) It does. And in one research project, high speed cameras recorded how the scent moves into the air.
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15 .(male student) Really?
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16 .(professor) Yes, when raindrops land on dry soil, that\'s really sponge-like or porous. Little bubbles form. And when these bubbles rise up and pop, they release tiny droplets with that familiar scent.
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17 .The smell is often strongest when it\'s raining for the first time in a long while. And Ned, it was quite observant of you to make the connection between this and the fragrance of the clay you work with.
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18 .(male student) Well, that smell is really, it\'s really noticeable.
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19 .(professor) And in case you\'re interested, the smell often detected before rain. That comes from the chemical ozone. Ozone is produced by lightning interacting with gases in the atmosphere.
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20 .We\'ll be discussing that later in class but an interaction like that, it\'s the sort of thing you can research. Remember, you do still need a topic for your next research paper.
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21 .(male student) Great suggestion, thanks.
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22 .(professor) Okay, then hope to see you at the gallery.
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