Page:Outlines of Physical Chemistry - 1899.djvu/71

 observations on gaseous and liquefied carbon dioxide. In this figure the pressures are denoted in atmospheres, the zero point of the ordinate starting at 45 atmospheres. The abscissae-axis is marked off to denote the volumes in thousandths of the normal volume (that is, of the volume occupied by the gas at 0°C, and under the pressure of one atmosphere). Let us study the isotherm 18 # l o 0. From a to b the pressure increases and the volume diminishes. From b to c liquefaction takes place — the pressure remains

��almost constant, whilst the volume gradually diminishes. This volume corresponds to a non-homogeneous mixture of liquid and gas. After complete liquefaction an increase of pressure produces only a very slight diminution in volume, consequently the product pv rises very rapidly (curve c d).

At a slightly higher temperature, 21 # 5° for instance, the horizontal line is shorter, that is, the period of lique- faction is reduced. At 31° the horizontal line has dis-

e a

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