224 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME Allowances must he made for expansion and contraction in steam pipes and water pipes in buildings, in long pipe lines, in the construction of bridges, paving, and railroad tracks. Expansion in pipes is permitted by the use of tight sleeves, within which the pipes can work back and forth. Pipe lines use large loops, whose change in curvature will take care of expansion and contraction. Rivets in steel girders and sheets are put in place while hot, partly because they are softer and thus easier to work at high temperatures, but also because they contract on cooling to form very tight joints. Cracks are left between sections in concrete paving; these cracks are generally filled with tar. On a very hot day the tar may sometimes be seen to bulge up at the cracks due to the expansion of the concrete. Bimetallic-strip thermostats, widely used in temperature-regulating devices, consist of two strips of metal with different coefificients of expansion, such as brass and steel, fastened together so that the strip is bent when it is heated. Some thermometers built into ovens work on this principle. Thermostats of certain types utilize bimetallic strips whose motion closes electric contacts or air-pressure lines, which thus regulate the heat supply or the electricity for house-heating systems, hot-water heaters, refrigerators, and electric irons. The coefficients of linear expansion of a few common substances are shown in the following table: Material Ratio of Increase in Length to Length at 0° C, per Degree C. Aluminum 0.0000255 Concrete 0.0000168 (varies with composition) Copper 0.000014 Steel 0.000013 (varies with composition) Lime 0.000009 (varies with composition) glass (ordinary) . . Platinum 0.000009 Borosilicate glass .... 0.000003 (varies with composition) Ordinary glass has to be annealed after it has been heated to a high temperature. Annealing is accomplished by cooling the glass slowly so that one portion will not cool more rapidly than another, thus avoiding strains in the glass which would result in the glass breaking too easily. Borosilicate glass has a much lower coefficient of expansion than ordinary glass and may therefore be heated to high temperatures and be cooled quickly without fracture. For that reason it has been used in making oven-ware and laboratory apparatus. Utensils now made of specially treated low-expansion glasses may be used for cooking foods over a free flame.
THE PHYSICAL PROPERTIES OF SOLIDS 225 Platinum has been used when it was necessary to fuse a metal into lime glass because it has the same coefficient of expansion as this glass. Metals of a different coefficient of expansion would cause cracks to form around the seal when it was cooled. Satisfactory cheap alloys such as kovar, which consists of a mixture of iron, nickel, cobalt, and manganese, and "Dumet" wire, consisting of a copper-clad steel wire, are now used to replace platinum in the manufacture of electric-light bulbs and other products in which metal wires must be sealed in glass. STUDY QUESTIONS 1. How does a true solid differ from a vitreous solid? 2. Give five examples of true solids and of vitreous solids. 3. Mention two respects in which water shows unusual properties. 4. State Le Chatelier's principle and illustrate it with several examples. 5. Explain the effect of adding heat to, or subtracting heat from, a mixture of ice and water. 6. Mention three advantages of dry ice as a refrigerant. 7. Mention some of the consequences of the fact that water has its maximum density at 4° C. 8. Explain how the principle of the expansion of metals is used in a thermostat. 9. Why is borosilicate glass better than lime glass for cooking-utensils? 10. Explain how the formation of ice moderates winters. IL Give the principle back of each of the applications listed in the summary.