742 JOHN JOHNSTON 



of the law of mass action, as the calculations made by Stieglitz 

 show/ 



A closely related point is that a compound resulting from a 

 reaction involving a volatile component will not be formed unless 

 the concentration — in other words, the partial pressure — of that 

 component is above a certain limiting value, this value being 

 dependent on the conditions, on the temperature especially. An 

 illustration may serve to make this clearer. If we wish to keep 

 liquid water at 200° we must have it in a closed vessel capable of 

 withstanding the vapor pressure of water at that temperature, 

 which amounts to about 15 atm. (225 lbs, per sq. inch); likewise at 

 300° the vessel would require to be strong enough to support an 

 internal pressure of 100 atm.; but if the vessel should leak, either 

 because it was not strong enough or because it was not tightly 

 closed, the vapor would escape and the liquid water would dis- 

 appear. Similarly we can keep Ca(0H)2 as hydroxide at 550° in 

 steam at i atm., but at 750° it would be necessary to have a pres- 

 sure of water vapor of about 15 atm. (and at 950°, about 100 atm.) 

 if we wish to retain it all as hydroxide and to have no oxide present; 

 in other words, if we should wish to prepare Ca(0H)2 from CaO 

 at a temperature of 750°, we could do so only by having continu- 

 ously a pressure of at least 15 atm, of water vapor above it. 



It may happen, therefore, that the product which actually sepa- 

 rates in certain cases will be determined by the magnitude of the 

 pressure of water vapor at the time of separation. This serves to 

 account for the fact that the biotite of many deep-seated igneous 

 rocks is replaced in their effusive forms by olivine and leucite 

 (which are in the aggregate chemically equivalent to biotite from 

 which water has escaped) — a relation which has been brought out 

 by Iddings, Backstrom, and others. Similarly, muscovite is found 

 only in granitic rocks consolidated at depth, while in their surface 

 equivalents the water has in part escaped and the potash has 

 entered feldspar and biotite. 



' J. Stieglitz, "The Relations oE Equilibrium between the Carbon Dioxide of the 

 Atmosphere and the Calcium Sulphate, Calcium Carbonate, and Calcium Bicarbonate 

 of Water Solutions in Contact with It," Carnegie Institution of Washington, Publication 

 No. 107 (1909) {The Tidal and Other Problems, by T. C. Chamberlin et al.), pp. 235-64. 



