The difference between a liquid and a gas is obvious【66】the conditions of temperature and pressure commonly found at the surface of the Earth. A liquid can be kept in an open container and f【67】it to the level of a free surface. A gas forms no free surface but tends to diffuse throughout the space available ; it must【68】be kept in a closed container, as in the【69】of a planet's atmosphere. The distinction was a prominent feature of early theories【70】(describe) the phases of matter. In the nineteenth century, for example, one theory maintained that a liquid could be 'dissolved' in a vapor without losing its identity, and another theory held that the two phases are made up of different kinds of molecules (分子). The theories now prevailing t【71】a quite different approach by emphasizing【72】liquids and gases have in common. They are both forms of matter【73】have no permanent structure, and they both flow easily. They are fluids. The f【74】similarity of liquids and gases becomes clearly apparent when the temperature and pressure are raised somewhat. S【75】a closed container partially filled with a liquid is heated. The liquid expands, or in other【76】, becomes less dense; some of it evaporates. In contrast, the vapor above the liquid surface becomes dense as the evaporated molecules are added【77】it. The combination of temperature and pressure【78】which the densities become equal is c【79】the critical point. Above the critical point the liquid and the gas can no longer be dis【80】; there is a single, undifferentiated fluid phase of uniform. density. (71)