填空题

Hardly anyone pays much attention to the air. It is (36) , and we never feel it unless a strong wind (37) along, blowing things around. But even though we are rarely (38) of it, the air is always around us. It is an amazing and very powerful substance, with (39) that few people know anything about. We live in a world that is always in (40) . Even still air and ice and solid rock are (41) with movement. This movement is the dancing of the tiny molecules that make up all matter. The molecules, in mm are made up of still tinier atoms, which are the smallest units of (42) . In air, which is a mixture of gases, this dance takes the form of a flee-for-all, in which (43) molecules move at a speed of nearly 250 miles an hour and bump into other molecules five thousand million times a second. (44) . Usually we don’t feel this pressure because it is equaled by pressure from inside our bodies. In fact, (45) . At this very minute, your body is supporting 14 tons of air pressure. (46) . We can rarely see this water, but it is there just the same, like a huge ocean over our heads.