In the air we breathe, the water we drink, and the bread we eat we have examples of the three different forms or states that matter can assume; namely, gaseous, liquid, and solid. A solid is a body which at ordinary temperatures and under slight pressures, does not change its shape. If the shape is changed under these conditions, the body is a fluid. Fluids may be divided into two classes. Those that retain a definite surface on being poured into a vessel are liquids, while those that have a tendency to expand indefinitely are gases.
Bodies form an almost continuous gradation from the most rigid solid to the most tenuous gas, and the above classification may be extended as in the following table
Fluids flow; it is commonly supposed that solids never do. This is not strictly true, since it has been shown that under certain pressures solid bodies also flow. The state of matter is largely determined by conditions of temperature and pressure. A stick of sealing wax fastened at one end so that it will stand horizontally and having a two-pound weight attached to the other end will become permanently bent in a short time; an asphalt pavement on a sloping street will flow down hill during a hot day; and a bullet placed upon a cake of shoemaker's wax, resting upon two corks in a dish of water, will in a few months pass entirely through the wax, while the corks will pass upward into it. All these are examples of what are called solid bodies, yet under the proper conditions they are seen to flow.
