The Romans observed that, when volcanic ash mixes with water and then cools, it gets extremely hard and almost impossible to break up.
罗马人观察到,当火山灰与水混合然后冷却,火山灰变得非常坚硬,几乎不可能分解。
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Meanwhile, its place is taken by hotter intergalactic gas from farther out on the edge of the galaxy, which cools as it is compressed and flows into the galaxy.