PRESSURE IN FORMATION OF ROCKS AND MINERALS 743 



A noteworthy point in this connection is that water vapor — or, 

 for that matter, any volatile component — may, and does, act in a 

 way precisely analogous to any non-volatile component, the only 

 difference being that it in general requires pressure in order to hold 

 it on the job, so to speak. As an illustration, let us consider the 

 equilibrium diagrams in Fig. 2, I for the system LiNOj-KNOs, 

 II for the system H.O-KNO3, III for the system H^O-CrOj. In 

 I, L is the melting-point of pure LiNOj, which by admixture of 

 KNO3 is lowered along the line LE: this line therefore represents 

 the equilibrium between solid LiNOj and liquid mixtures of LiNOj 

 and KNO3 of various compositions; in other words, it is the solu- 

 bility curve of LiNOj in these mixtures. Similarly KE represents 

 the equilibrium between solid KNO3 and liquid. These two curves 

 meet in E, the so-called eutectic point, at which point (132°) a 

 liquid of the composition 45 per cent LiNOj, 55 per cent KNO3 

 freezes to an agglomerate of the same composition. In II, point / 

 is the freezing-point of water, and IC the freezing-point of solutions 

 of KNO3; in other words, IC is the equilibrium line along which 

 ice is in equilibrium with mixtures of H2O and KNO3 of various 

 compositions. The line KC represents the equilibrium between 

 solid KNO3 and mixtures of KNO3 and H2O; the end nearer C is 

 the ordinary solubility curve of KNO3 in water, while toward K 

 it would be more logically called the solubility curve of II2O in 

 KNO3. The curves meet in C, which in the case of aqueous solu- 

 tions is usually called the cryohydric point. These two systems 

 are thus obviously altogether similar, the sole difference resulting 

 from the difference in the vapor pressures of H2O and LiN03 in 

 relation to the prevailing atmospheric pressure; namely, that 

 whereas at the temperatures concerned the vapor pressure of LiNOj 

 is inappreciable, that of water ranges from some millimeters up to 

 many atmospheres. The equilibrium curve (III, Fig. 2) for the 

 system H20-Cr03 is included because in form it resembles very 

 closely that for LiNOj-KNOj ; the branch of the curve on the water 

 side is much longer than in the case of H2O-KNO3, corresponding 

 to the much greater solubility of Cr03 in water at and below 0°. 



At each point on the curve CK the liquid in equilibrium with 

 solid KNO3 has a definite composition and therefore a definite vapor 



