_ PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 487 
Importance of the rate of transformation.—The point in question 
is that the factor which determines the actual experimental result 
may be the rate at which equilibrium is established under the 
particular conditions. We must therefore always ascertain 
whether the observed result is, or is not, due to a laziness—as 
opposed to an inability—to react; in other words, we must dis- 
tinguish carefully between cases of true and apparent, or false, 
equilibrium. Lack of care in this matter may lead us to consider 
a transformation to be monotropic or a given form to be stable under 
conditions where it really is metastable, its persistence in either 
case being merely the result of a slow rate of transformation. 
As an illustration, take the system represented by the scheme 
2H,+0,52H,0O, a system which has been so thoroughly investi- 
gated’ that we know the conditions of equilibrium at all tempera- 
tures up to 2,600°. These conditions are such that at all tempera- 
tures below 1,000°, the pressures of oxygen and hydrogen which can 
exist in real equilibrium with water are infinitesimally small;? 
hence, if real equilibrium obtained, it would be altogether impos- 
sible to preserve in presence of water an appreciable amount of a 
mixture of oxygen and hydrogen. In other words, this system is, 
theoretically, absolutely unstable; practically, however, hydrogen 
and oxygen may be left in contact together with water for an 
unlimited time at ordinary temperatures, and still show no signs 
of reaction, which begins only at fairly high temperatures, i.e., 
when its velocity becomes appreciable. 
This case is instructive also in showing that there is no necessary 
parallelism whatever between the “‘affinity”’ of two substances for 
one another (which is measured by the change of free energy accom- 
panying a reaction) and the rate of reaction under any particular 
conditions. Thus the above reaction does not go (except at high 
temperatures), although the energy change accompanying it is 
one of the largest known; on the other hand, many reactions with 
comparatively feeble energy changes go quite readily. Moreover, 
See Nernst, Theoretische Chemie; Bjerrum, Z. physik. Chem., LX XIX (1912). 
2 Thus the extent of dissociation is of the order of 1o—25 per cent at 25°, 1o—5 per 
cent at 700°, and does not reach a magnitude which is directly measurable until about 
1,200°, where it is about 0.02 per cent. 
