222 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME 3. When a snowball is squeezed, it is compacted into ice because some of the snow melts as pressure is applied, thus relieving the pressure because of the smaller volume occupied by liquid water than is occupied by the solid water or snow. When the pressure is released, the reverse process takes place; the water freezes and binds the snow together. Liquid films are produced on ice by the pressure produced against the ice by the weight of the skater. These films offer less resistance than solid ice offers and thus enable the skater to glide over the ice very readily. The glazed tracks left on snow by a sleigh are due to a similar melting from pressure and refreezing after removal of the pressure. " Regelation " is the term applied to the melting of ice under pressure and the subsequent freezing upon release of the pressure. Glaciers are able to "flow" around obstructions by this process. Substances like sulfur which increase in volume when melted cannot be melted by pressure. This principle of Le Chatelier is one of the most far-reaching generalizations in the whole field of Science. Applications of it will be noted later in systems of chemical and physiological equilibria. Heat of Fusion Is an Important Factor in Refrigeration. The ice refrigerator illustrates an application of the high heat of fusion of ice. The heat in the refrigerator is removed by the melting of the ice. Some people wrap ice in burlap or newspapers to make it last longer, not realizing that it is the melting of the ice that keeps the refrigerator cool. Such a practice is dangerous because the temperature of the refrigerator is not kept sufficiently low to prevent the multiplication of bacteria and the false sense of security causes the owner to be less careful about cooking the foods before use than he would otherwise be. "Dry ice" is the common name for solid carbon dioxide. In recent years dry ice has replaced water ice for many purposes because of the following advantageous properties which it possesses: 1. Solid carbon dioxide does not melt but changes directly into a gas. This change from a solid state to a gaseous state without passing through a liquid state is called sublimation. The rotting of wood in refrigerator cars and other places where water comes into contact with wood is thus eliminated when "dry ice" replaces water ice. The fact that no liquid is formed when "dry ice" melts is applied in packing small home containers of ice cream. 2. The gas formed when dry ice sublimes prevents bad odors in refrigerating cars and actually acts as a preservative by killing bacteria.
THE PHYSICAL PROPERTIES OF SOLIDS 223 3. Salt is unnecessary for obtaining freezing temperatures with dry ice, thus eUminating the salt water dripping from railroad refrigeration cars which has produced considerable corrosion of the rails. In the preparation of dry ice, purified carbon dioxide is liquefied by means of pressure. The liquid carbon dioxide is then allowed to evaporate so rapidly that a portion of the liquid is solidified as a result of the decrease in temperature caused by the absorption of heat in this process. Solids Change in Volume When Heated. We have already seen that a change in volume is always brought about when a gas, liquid, or solid is heated. The majority of solids increase in volume when heated, but in a few cases the solid state occupies a greater volume than the liquid state of a substance; this is one of the unusual properties of water. When water is cooled, it contracts until it reaches about 4° C. Below this temperature it expands upon cooling. The density of ice compared with water is 0.917. This means that ice is nearly one-tenth lighter than water; only about one tenth of an iceberg floats above the surface of the water. In some respects it is unfortunate that water expands upon freezing because it causes our water pipes and automobile radiators to burst in winter. The expansive force exerted by the freezing of water is so great that few containers are able to withstand it. On the other hand, the principle of the expansion of water on freezing has many valuable applications in nature. It has already been pointed out that surface rocks are split apart as the moisture in them is frozen in the winter. If ice were more dense than water, it would sink in the ponds and rivers as fast as it was formed in the winter ; such bodies of water would freeze solid and most of their animal life would be destroyed. The oceans, lakes, and rivers would all freeze from the bottom up, and summer melting would be confined to a little slush on top. The fact that the maximum density of water occurs at about 4° C. also enters into this problem. When water reaches the temperature of about 4° C. it begins to expand and therefore remains at the surface of the body of water. The temperature of deep bodies of water does water cooled not change much because both warm water and also below 4° C. stay at the top. Cast iron, antimony, and its alloy type-metal (82 per cent lead, 15 per cent antimony, 3 per cent tin) are among the few substances that expand as they solidify. Coins made from copper, gold, silver, or nickel must be stamped on a metal disk with a heavy die rather than be cast because these metals contract as they solidify.