Some bodies are _solid_; as ice, iron, wax. Others are _liquid_; as water, mercury, oil. Still others are in the state of _gas_; as steam, air, and illuminating gas. Further we notice that the same substance may be found in any one of the three states. For example water may be either ice, water or steam; that is, either a solid, a liquid, or a gas. Most persons have heard of _liquid air_ and possibly some know of _ice air_, _i.e._, air cooled until it not only liquefies, but is solidified. On the other hand, iron may be melted and, if heated hot enough, may be turned into iron vapor. In fact most substances by heating or cooling sufficiently may be changed into any one of the three states. Before defining the three states, let us consider the _structure_ of matter. This may help us to answer the question: How is it possible to change a hard solid, such as ice, into a liquid, water, and then into an invisible gas like steam? This is explained by the molecular theory of matter. =6. The Molecular Theory of Matter.=--It is believed that all bodies are made up of very small particles called _molecules_, and that these instead of being packed tightly together like square packages in a box, are, strange as it may seem, very loosely packed even in solids and do not _permanently_ touch their neighbors. The size of these molecules is so minute that it has been estimated that if a drop of water could be magnified to the size of the earth, the molecules magnified in the same proportion would be in size between a baseball and a football. The air and all other gases are believed to be made up of molecules in _rapid motion_, striking and rebounding continually from one another and from any objects in contact with the gas. =7. States of Matter Defined.=--These ideas of the structure of matter assist us in understanding the following definitions: _A solid is that state of matter in which the molecules strongly cling together and tend to keep the same relative positions_. (This of course follows from the tendency of a solid to retain a definite form.) _A liquid is that state of matter in which the molecules tend to cling together, yet move about freely._ Hence a liquid takes the form of any vessel in which it is placed. _A gas is that state of matter in which the molecules move about freely and tend to separate indefinitely._ Hence a gas will fill any space in which it is placed. =8. Effect of Heat on Matter.=--It is further believed that when a body is heated, that the action really consists in making its molecules move or vibrate faster and faster as the heating progresses. This increase of motion causes the molecules to push apart from one another and this separation of the molecules causes an expansion of the body whether it be solid, liquid, or gas. Fig. 1 shows the expansion of air in an air thermometer. Fig. 2 shows the expansion of a solid on heating.