510 JOHN JOHNSTON AND PAUL NIGGLI 
In a completely fluid system the influence of pressure on the 
position of equilibrium will not in general be large, because the 
volume change AV is usually small. In gaseous systems the direc- 
tion of the effect can readily be predicted, since the volume of a 
gas (at constant temperature and pressure) is a direct measure of 
the number of molecules of it present. Therefore pressure is with- 
out effect if the total number of molecules remains unchanged by 
the reaction; otherwise, it displaces the position of equilibrium 
toward the side with the smaller total number of molecules. 
In heterogeneous systems liquid-gas and solid-gas the pressure 
changes the concentration in the gas phase, and in this way may 
have very considerable effect on the position of equilibrium. 
The action of unequal pressure, however, on a heterogeneous 
equilibrium is of an entirely different order of magnitude. If a 
system solid-liquid or solid-gas is compressed in such a way that 
the fluid phase can escape continuously, the reaction will be driven 
in one definite direction. For example, if the system 
RCO,+Si0, > RSiO,+ CO, 
is compressed in such a way that the CO, escapes, RSiO, will always 
be formed. Then, too, unequal pressure influences greatly the 
solubilities of solids, and thus may entirely change the relative 
concentrations of the solid substances in the fluid phase. 
The foregoing paragraphs provide a basis for the observation 
that metamorphosed rock is usually denser than that from which 
it was formed, for the so-called Volumgesetz of the Germans. To 
this rule there are many exceptions; the extent to which it holds is 
about what we might expect; for while pressure tends to produce 
denser material, it may not actually do so if the rate of reaction is 
very small under the particular conditions. Moreover, we cannot 
safely conclude from measurements of specific volume made at 
ordinary temperature (as all of them hitherto have been) that the 
change of volume under the actual conditions of transformation 
was the same in magnitude or even in sign; for, as we have already 
pointed out, a difference (such as may easily occur) between the 
coefficients of expansion of the substances concerned may cause a 
reversal of sign of the volume change, especially when the latter is 
small. 
