properties—volume, density, weight, etc., in common. Other properties, such as malleability and ductility, affect groups or classes of matter—chiefly the metals.
Cohesion is a somewhat archaic term denoting molecular attraction. In solids, the cohesion is usually strong, so that more or less force is required to sunder the mass—that is, to separate the molecules. The resistance of cohesion is usually expressed in such terms as tension, torsion, shearing, etc. In the liquefaction of a solid, or the vaporization of a liquid, the force employed to overcome cohesion is measured in terms of heat. For instance, in the liquefaction of ice, about 147 times as much heat is required to change ice at 32° to water at 32° as will raise the temperature of the same weight of water one degree Fahrenheit in temperature. In the case of liquids, the cohesion seems to be slight, inasmuch as the molecules possess a considerable mobility. Nevertheless, they are held together by a powerful force. Thus, the heat used in converting one pound of water at 212° F to a vapor at 212° F would raise 967 pounds of water one degree in temperature. Measured thus, in terms of heat units, great power is required to overcome molecular attraction. In the case of gases, not only is the molecular attraction negligible, but the molecules apparently repel one another.[1] Perhaps it is more nearly correct to say that they diffuse themselves throughout the space which contains them. In other words they apparently cease to possess molecular attraction.
Crystallization is a form of molecular attraction which indicates that the molecules possess a certain kind of polarity, usually assuming regular geometric forms. Frost and snowflakes frequently exhibit marvelous forms, infinite in variety but regular and similar in construction. The study of these forms is one of increasing importance in weather science.
Expansion-contraction is a property of matter true in its ordinary forms. The volume of a substance increases when it is heated and contracts with cooling. Thus, iron will increase- ↑ In mechanics the repellent force of the water vapor, usually called steam, is termed “pressure” and is rated in “pounds per square inch,” or in “atmospheres” of 14.7 pounds per square inch. In meteorology the repellent force is expressed sometimes as tension, but more commonly as pressure.
