9. =Liquids.=--It is the grand characteristic of a liquid that its particles change their relative position from the slightest causes. It is in strong contrast with solids in this respect. When you move any portion of a solid body you move all the other portions of it, and generally in the same direction. But a body of liquid can not be moved all together as one body except by confining it; as, for example, in the case of a water-pipe or a syringe. And then, the moment that the water can escape, the particles use their liberty of altering their relative position. As wind and other agents act continually upon water, no particle stays for any length of time in the neighborhood of the same particles. "Unstable as water" is, then, an exceedingly significant expression. Water is never at rest. A particle of it may at one time be floating on the surface of the ocean, and at another be in depths beyond the sounding of man. It flies on the wings of the wind, falls in the rain, runs in the stream, is exhaled from a leaf, trembles in the dew-drop, flows in the blood of an animal or in the sap of a plant, and is always ready to be jostled along in its ever-changing course. 10. =Gases.=--The particles of gaseous or aeriform substances move among each other even more freely than those of a liquid. Air, therefore, is more unstable and restless than water. Even when the air seems to be perfectly still its particles are moving about among each other. You can see this to be true if you darken a room, leaving a single shutter a little open. Where the light enters you will see motes flying about in every direction, which would not be the case if the air were really at rest. The particles of air have a greater range of travel than those of water; for the sea of atmosphere which envelops the earth rises to the height of about fifty miles. How far water rises in its evaporation we know not; but it is not at all probable that it rises to the uppermost regions of the atmosphere. 11. =Filling of Spaces by Liquids and Gases.=--It is the freeness with which the particles of liquids and gases move among each other that enables them to insinuate themselves into spaces every where. They are ever ready to enter into any substances which have interstices or pores of such size as will admit them. There are mingled with the grains of the soil not only water, but air and gases. These are present also in all living substances, both vegetable and animal. Water is the chief part of sap and of blood, and air and gases always go with water. Part of the air that we breathe in enters the blood in the lungs, and courses with it through the system. The fishes could not live in water if there were not air mingled with it. This can be proved by experiment. If you put a fish into a close vessel it will soon die, because it uses up all the air that is in the water. In an open vessel the fish is kept alive by the constant accessions of fresh air to the water.