PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 511 
As an application of the law of mass action to a very simple 
case, consider the dissociation of calcium carbonate according to 
the equation 
CaCO,;sCaO+CO,. 
We will presume that the reaction takes place in the gaseous phase: 
the equilibrium constant is then given by the equation 
(CaO) (CO,) _ C 
(CacOjr =” 
the quantities in parentheses denoting the partial pressure at 
equilibrium of the substance written within the parentheses." 
Now CaO and CaCO, are always present in the solid phase, from 
which it follows that at any one temperature (CaO) and (CaCO,) 
are constant; whence we have (CO,)=K. In other words the 
pressure of CO, in equilibrium with a mixture of CaO and CaCO, 
’ is independent of the amounts of the solids present, and has a 
definite value for every temperature;? its variation with tempera- 
ture being given by equation ITI. 
E. Baur’ has investigated a system of considerable geologic 
interest, namely, the reaction 
SiO.+4HFs SiF,+2H.,0O 
which takes place when aqueous hydrofluoric acid is distilled in 
presence of excess of silica. Under these conditions SiO, is solid, 
the other three substances gaseous; therefore at definite tempera- 
ture and pressure we should find that the expression 
(SiF,) (HO) _ 
(1S) aia 
tIt may be objected that it is absurd to talk about the vapor pressure of a sub- 
stance such as CaO; we have, however, every reason to believe that such pressures, 
though infinitesimal, are real and perfectly definite. Application of the mass law to 
the vaporization of CaO, that is, to the reaction CaO (solid) > CaO (vapor), gives 
(CaO) 
CaO (solid) — 
stant and therefore at constant temperature (CaO)=K”’. 
the relation K’; but since excess of solid is present, CaO (solid) is con- 
2 These values may be found in Johnston, Jour. Am. Chem. Soc., XXXII (1910), 
938. 
3Z. physik. Chem., XLVIII (1904), 483. 
