Machinery's Handbook, 31st Edition
Weight of Piles 381 Molecular Weight.— The smallest mass of a chemical combination which can be con ceived of as existing and yet preserving its chemical properties is known as a molecule . The molecular weight of a chemical compound is equal to the sum of the atomic weights of the atoms contained in the molecule, and is calculated from the atomic weights, when the symbol of the compound is known. See Table 1 on page 368 for atomic weights. The atomic weight of silver is 107.88; of nitrogen, 14.01; and of oxygen, 16; hence, the molec ular weight of silver-nitrate, the chemical formula of which is AgNO 3 equals 107.88 + 14.01 + (3 3 16) = 169.89. Mole or Mol.— This term is used as a designation of quantity in electro-chemistry to in- dicate the number of grams of a substance equal to its molecular weight. For example, one mol of silver-nitrate equals 169.89 grams, the molecular weight of silver-nitrate being 169.89. Air.— Air is a mechanical mixture composed of 78 percent, by volume, of nitrogen, 21 percent of oxygen, and 1 percent of argon. The weight of pure air at 32 °F (0 °C), at an atmospheric pressure of 29.92 inches of mercury (760 mm mercury or 760 torr) or 14.70 pounds per square inch, is 0.08073 pound per cubic foot. The volume of a pound of air at the same temperature and pressure is 12.387 cubic feet. The weight of air, in pounds per cubic foot, at any other temperature or pressure may be determined by first multiply - ing the barometer reading (atmospheric pressure in inches of mercury) by 1.325 and then dividing the product by the absolute temperature in degrees F. The absolute zero from which all temperatures must be derived in dealing with the weight and volume of gases, is assumed to be minus 459.67 °F (273.15 °C). Hence, to obtain the absolute temperature, add to the temperature observed on a regular Fahrenheit thermometer the value 459.67. Alligation.— Alligation or “the rule of mixtures” are names applied to several rules of arithmetical processes for determining the relation between proportions and prices of the ingredients of a mixture and the cost of the mixture per unit of weight or volume. For example, if an alloy is composed of several metals varying in price, the price per pound of the alloy can be found as in the following example: An alloy is composed of 50 pounds of copper at $1.70 a pound, 10 pounds of tin at $4.05 a pound, 20 pounds of zinc at $0.99 a pound, and 5 pounds of lead at $1.10 cents a pound. What is the cost of the alloy per pound, no account being taken of the cost of mixing it? Multiply the number of pounds of each of the ingredients by its price per pound, add these products together, and divide the sum by the total weight of all the ingredients. The quotient is the price per pound of the alloy. Example: The foregoing example would be worked out numerically as follows: Total cost of materials: 50 × 1.70 +10 × 4.05 + 20 × 0.99 + 5 × 1.10 = $150.80 Total weight of metal in alloy: 50 + 10 + 20 + 5 = 85 lbs. Price per pound of alloy = 150.80 ÷ 85= $1.77, approximately. Earth or Soil Weight.— Loose earth has a weight of approximately 75 lb/ft 3 (1200 kg/m 3 ) and rammed earth, 100 lb/ft 3 (1600 kg/m 3 ). Composition of Earth Crust: The solid crust of the earth, according to an estimate, is composed approximately of the following elements: oxygen 44 to 48.7 percent; silicon 22.8 to 36.2 percent; aluminum 6.1 to 9.9 percent; iron 2.4 to 9.9 percent; calcium 0.9 to 6.6 percent; magnesium 0.1 to 2.7 percent; sodium 2.4 to 2.5 percent; potassium 1.7 to 3.1 percent. Loads on Soils and Rocks: The bearing capacities of soils and rocks is useful in structural engineering and also of value under certain conditions in connection with the installation of very heavy machinery requiring foundations. The ultimate resistance of various soils and rocks will be given in tons per square foot: natural earth that is solid and dry, 4 to 6 tons; thick beds of absolutely dry clay, 4 tons; thick beds of moderately dry clay, 2 tons; soft clay, 1 ton; gravel that is dry, coarse, and well packed, 6 to 8 tons; soft, friable rock and shales, 5 to 10 tons; sand that is compact, dry, and well cemented, 4 tons; natural sand in a clean dry condition, 2 to 4 tons; compact bedrock, northern red sandstone, 20 tons; compact bedrock, northern sound limestone, 25 tons; compact bedrock granite, 30 tons.
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