Page:International Library of Technology, Volume 93.djvu/71

 = 12$1⁄2$ pounds, and the corresponding point is G, vertically above 8 and on a horizontal line passing midway between 10 and 15. In the same way, when v = 7 cubic feet, p = 100 ÷ 7 = 14$2⁄7$ pounds v = 6 cubic feet, p = 100 ÷ 6 = 16$2⁄3$ pounds v = 5 cubic feet, p = 100 ÷ 5 = 20 pounds v = 4 cubic feet, p = 100 ÷ 4 = 25 pounds v = 3 cubic feet, p = 100 ÷ 3 = 33$1⁄3$ pounds v = 2 cubic feet, p = 100 ÷ 2 = 50 pounds These volumes and their corresponding pressures are then laid off on the diagram, locating the successive points F, E, D, C, B, and A. If, now, a smooth curved line is drawn through the points thus located, the resulting curve KA is called the isothermal-compression curve. The points on this curve from K to A represent all the successive states of the air during its compression.

75. If 2 cubic feet of air at a pressure of 50 pounds, absolute, is allowed to expand, and heat is added continually during the expansion, so as to keep the air at constant tem- perature, it will pass through the same states as those indi- cated by the curve in Fig. 13, but in reverse order. That is, the curve AK will represent the successive states of the air. Hence, the curve AK is called the isothermal-expansion curve. It is now evident that, for any given weight of air, the curves of isothermal expansion and compression are identical. This is further proved, in the examples given, by the fact that the product of the pressure and the volume is the same during both expansion and compression.

76. Adiabatlc Expansion and Compression. — Suppose that a quantity of gas is confined in a non-conducting cylinder, that is, a cylinder made of a material that will not conduct heat or permit the passage of heat by radiation. Under these conditions, if the gas is compressed or if it is allowed to expand, it will change its state without losing any heat to, and without receiving any heat from, outside bodies. The gas is then said to undergo adiabatic compression or adiabatic expansion.