| The American Heritage® Book of English Usage. |
A Practical and Authoritative Guide to Contemporary English. 1996.
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4. Science Terms: Distinctions, Restrictions, and Confusions
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| § 26. fluid / liquid / gas |
| In science, the use of the word fluid applies to both liquids and gases. A fluid is a substance that does not have a fixed shape, and one whose molecular constituents move freely past one another. Thus, fluids take on the shape of their containers. The distinguishing feature between a liquid and a gas is that a liquid is a fluid that has a relatively fixed volume, and a gas is a fluid that does not have a fixed volume: gases can be compressed and can expand to fill a container entirely. | 1 |
| One exception to this scientific use of fluid is bodily fluids. Here the word fluid applies to liquids only. | 2 |
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| The American Heritage® Book of English Usage. Copyright © 1996 by Houghton Mifflin Company. All rights reserved. |
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