PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 513 
however, may be left out of account, since it plays no direct part 
in the reaction). 
We have then the equilibrium equations between these quanti- 
ties 
[Ba++] [SO'"]=K, and [Bat++][CO7]=K, 
where A, and K, are the solubility products of BaSO, and BaCO,, 
respectively; and therefore, since [Bat*] is common to both, 
; [SO’“)/[CO3]=Ki/K.=K . 
In words, the ratio of the concentrations of sulphate ion and car- 
bonate ion is constant. Therefore, if, after equilibrium has been 
attained, we add SO’; (as Na.SO,), BaCO, is transformed into 
BaSO,, until this ratio reattains its constant value; conversely the 
addition of COZ causes the formation of BaCO, at the expense of 
the BaSO,. In each case the amount transformed is perfectly 
definite; and the state of equilibrium can always be calculated if 
K,, K., and the concentration of either SO’ or CO’! are known. If 
the conditions were such that the system should become saturated 
with respect to sodium sulphate or carbonate (or both), the equa- 
tion determining the equilibrium would be somewhat more com- 
plicated; but the general result would be the same, namely, that 
the position of equilibrium is determined by the relative solu- 
bilities (in the particular medium) of such of the possible products 
of reaction (which include the original substances) as separate out 
as solid phases. 
On this basis we can readily see why it may be that carbonic 
acid may displace silicic acid from a solution of a silicate at low 
temperatures, while at high temperatures the silicic acid may dis- 
place the carbonic acid and regenerate the silicate; and that it is 
unnecessary to bring in the conception of affinity or strength of 
acids to account for the phenomena observed. Nor is it necessary 
to attribute the reversal in aqueous solution of reactions such as 
the above to the influence of pressure, as is frequently done. It is 
true that pressure is required in order to retain the volatile com- 
ponents, the concentration of which in the vapor phase is thus 
determined by the pressure; but it is limited to this more or less 
subsidiary réle. The predominating factor in determining the 
state of equilibrium in solution is temperature, which acts primarily 
