220 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME No System of Classification Is Adequate. The majorit>- of subst^uiccs can be classified definitely as gases, liquids, or solids, but there are a few substances that show intermediate properties; thus vitreous liquids show by X-ray analysis that there are at least temporary crystalline structures present. Several hundred liquids which show crystalline properties are known. Solids Have Definite Specific Heats. The specific heat of a substance is the ratio oj the amount of heat required to raise the temperature of one gram of the substance one degree centigrade to the amount similarly required for water. A CALORIE is the amount of heat required to raise the temperature of one gram of water from 15° to 16° C. Table of Specific Heats Water .... 1.00 Sand . Pine wood . . . 0.65 Iron Alcohol . . . . 0.60 Copper Ice 0.50 Zinc . Aluminum . . . 0.22 Mercury 0.19 0.113 0.094 0.093 0.033 The unusually high specific heat of water has a number of important applications. In a following section we shall see how the temperature of land areas near large bodies of water is kept equable by the breezes blowing from the water to the land areas. The temperature of large bodies of water changes very little during a hot day because of the high specific heat of water. On the other hand, large bodies of water do not cool very much at night for the same reason. Inasmuch as the specific heat of land areas is much less than that of water, they are more quickly heated and cooled than large bodies of water. Hot-water heating systems are possible because of the large amount of heat that is carried by the water and given off as the water slightly cools. The hot-water bottle is an application of the high specific heat of water. The same weight of iron would yield less than one-ninth as much heat for an equivalent temperature drop. The Heat of Fusion of Water Is Unusually High. When liquids solidify, heat is set free, and when solids melt, exactly equivalent amounts of heat are used up for equivalent amounts of the same substances. The amount of heat in calories required to melt one gram of a true solid is called its heat offusion. The freezing of water into ice liberates so much heat that winters are moderated and the advent of spring is delayed in the neighborhood of large bodies of water because of the heat absorbed by the ice as it melts.
THE PHYSICAL PROPERTIES OF SOLIDS 221 Heat of Fusion Calories PER Gram Sodium chloride 124 Ice 79.8 Aluminum 76.8 Copper 43 Tin 14 Lead 5.4 Le Chatelier's Law Is the Fundamental Law of Equilibria. When a mixture of ice and water is heated, the ice melts; and the water freezes when the mixture is cooled. Both processes successfully resist a change in temperature until either the ice or water is used up. This is an illustration of a general principle formulated by Le Chatelier, which in simple language merely states that when a system that is in equilibrium is subjected to a stress, the equilibrium is disturbed and a change takes place in the direction that tends to relieve the stress until the system once more reaches a state of equilibrium. An equilibrium can be subjected to a stress by altering the temperature or pressure of the system or the concentration of one or more of the substances contained in it. The application of this principle to physical equilibria may be illustrated as follows: 1. When a liquid in equilibrium with its vapor has heat added, part of the liquid evaporates, inasmuch as vaporization absorbs part of the added heat. If heat is removed from the system, part of the vapor condenses; and the heat thus evolved tends to compensate for the heat removed. 2. When a solid is in equilibrium with its liquid, the temperature of the system cannot be permanently altered without completely removing either the solid or the liquid. Thus ice and water will remain together without an increase in the amount of either phase as long as no heat is gained nor lost by the system. Ice will remain in water in a good thermos bottle for some time because little heat is allowed to enter the system. The temperature of a mixture of ice and water is 0° C, and it cannot be changed by either adding heat to, or removing it from, the mixture; when heat is applied, a portion of the ice melts and thus absorbs the heat; when heat is removed, a portion of the water freezes and thus evolves heat. A tub of water may be placed in a basement to prevent the freezing of nearby vegetables. The principle of this procedure is that the heat liberated as the water is first cooled and is then frozen will help to prevent the freezing of the vegetables, which should be protected in part by being covered with cloth or earth.