Ice, water, and steam represent the three states of liquid matter. A block of ice has a definite form and volume. Water has a free, level surface, but assumes the shape of the containing vessel. Steam has neither shape nor volume. Notice the steam escaping from a kettle or from the exhaust pipe of a power plant, and see how it tends to spread out when released from the containing vessel. Almost all substances can be transformed into a solid, liquid, or gaseous state by suitable changes in temperature. We may summarize the characteristic differences of these three conditions by saying that solids have permanent form and volume; that liquids have no permanent form, but have a definite volume; while gases have neither permanent form nor permanent volume.

All gases tend to spread out or diffuse themselves and this tendency causes them to exert considerable pressure equally against the sides of the vessels holding them. If a piston were attached to the side of a vessel, the gas would tend to push the piston out, provided the pressure of the gas on the inside was greater than that of the atmosphere on the outside. This is the case when a gas is compressed in a tank. The gas may be transferred from place to place intact, and then allowed to pass through pipes, to the place where its energy is to be utilized.