FLD JOHN JOHNSTON AND PAUL NIGGLI 
The actual determination of the various concentrations is very 
difficult, so that moderate concordance between the values of K is 
all that can be expected. At 104°, K is of the order of 2X10, at 
270° it is about 5107. This decrease of K with rise of tempera- 
ture shows that the equilibrium is displaced toward the left by rise 
of temperature, so that the reaction, as written above, is at these 
temperatures slightly exothermic. The influence of pressure is 
easily predicted; for, since we have 3 volumes on the right side, as 
compared with 4 volumes on the left (the volume of the solid silica 
is, in comparison, negligible), pressure will displace the equilibrium 
toward the right-hand side of the equation above. From the above 
it follows that, when such vapors escape from the magma, lowering 
of their temperature alone will result in a dissolution, instead of a _ 
precipitation, of SiO.; decrease of pressure, on the other hand, 
results in the production of quartz—an effect which will presumably 
predominate over that produced by decrease of temperature, since 
the volume change is large but the heat change accompanying the 
reaction comparatively small. 
Constant solubility product.—Again we can readily deduce from 
the mass-action law the experimental fact of the constancy of the 
so-called solubility product. Thus for the substance AgCl we 
find that 
(Eg) (CI ae 
where (Ag*) and (Cl7) represent the real concentrations of silver 
ions and chlorine ions in the solution; K is a constant, depending 
only upon the temperature. This relation holds true for any solu- 
tion in which silver ions and chlorine ions may be present together 
in contact with solid AgCl—no matter what other ions may also 
be present—provided always that (Ag*) and (Cl~) are understood 
to be the real concentrations of the respective ions, when equilibrium 
has been attained. 
As a specific example of the application of this principle to the 
determination of equilibrium conditions, we shall consider the 
reversible reaction 
BaSO,+Na.CO;= BaCO;+Na,SO, . 
There are present in the system the following molecular species: 
un-ionized BaSO, and BaCO,, Batt, SO'/, COZ (and Na™, which, 
