.2000年 一The difference between a liquid and a gas is obvious (26) the conditions of temperature and pressure commonly found at the surface of the Earth. A liquid can be kept in an open container and (27) it to the level of a free surface. A gas forms no free surface but (28) to diffuse throughout the space available; it must (29) be kept in a closed container, as (30) a planets atmosphere. The distinction was a prominent feature of early theories (31) the phases of matter. In the nineteent
