Page:Outlines of Physical Chemistry - 1899.djvu/70

 ��OUTLINES OF PHYSICAL CHEMISTBY

��critical constants. I shall refer only to the relation which exists between the critical temperature of a substance and its boiling point measured on the absolute scale.

The critical temperature is equal to twice the boiling point at a greatly reduced pressure (20 millimetres of mercury) or to 1'55 times the boiling point at normal atmospheric pressure.

Thus for chloroform the boiling point = (273 +61)° under normal pressure. /. the critical temperature = (278 + 61) x 1'55 = 518°.

Actual experiment gives the critical temperature as 588° (absolute scale).

��APPENDIX

Van deb Waals's Equation

Graphical Bepresentation.— To represent the pheno- mena studied in the preceding chapter, that is, the transi- tion between the liquid and gaseous states, we can make

use of a system of two rectan- gular co-ordinates and mark off volumes on the abscissae- axis and pressures as ordi- nates. For a gaseous mass maintained at constant tem- perature the correlative values of p and of v would determine the successive points of a rect- angular hyperbola if Boyle's law were absolutely exact. Fig. 5 represents such an hy- perbola ; it is an ideal isotherm of the products p v, and its equation is : p v = constant. But in reality the isotherms are quite different from an equilateral hyperbola, as can be seen from fig. 6, which represents a series of Andrews's

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