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FREDERICK BARRY 
very specific cause of variability affects these latter values, namely, the 
partial inactivation of the catalyst by heat. 
The empirical statement of van t'Hoff quoted above is very often 
referred to, and is commonly used as a rough indication of the degree 
in which the velocity of a reaction or of a complicated series of re- 
actions will be affected by temperature changes. The more com- 
plicated the phenomena, of course, the more likely it will be according 
to the theory of chance that the rule will hold. It is, however, only very 
roughly approximate; and, of course, must be discarded altogether if 
phenomena are under consideration which involve any variable energy 
interchange between the chemical system studied and its environment. 
It has, in fact, been the tacit assumption throughout the present 
discussion that isolated systems alone were under examination. The 
degree in which van t'Hoff's rule applies to physiological processes 
is determined first of all by the degree of uniformity in which the 
partially isolated cell is affected by environmental changes. That it 
applies at all, is a matter explainable only by the fact that in physio- 
logical reactions the amount of total energy change is, excepting for 
the influence of change in environmental conditions, very small. In 
consequence, as has been remarked, the equilibrium constant of any 
such reaction is relatively independent of the temperature. Since 
this constant is equal in any case to the ratio of the velocity constants, 
this must mean that these represent nearly the same temperature 
function; which signifies that in such cases the sum of all forces op- 
posed to that of the change in free energy varies in the same manner 
with change in temperature. The most influential cause of uniformity 
in physiological reactions beyond that brought about by small energy 
change, is to be found in the prevalence of enzymotic catalysis in all 
such reactions. The contact theory of catalysis requires — and the 
supposition is supported by many facts — that this influence, in the 
degree that it operates, reduces all speeds of reaction to the uniform 
speed of diffusion. This matter cannot be gone into here. It is 
pertinent only to remark that Euler's values for the temperature 
coefficients of enzyme reactions for ten degrees are smaller than those 
summarized by van t'Hoff; which in accordance with theory is what 
would be anticipated.^^ If, therefore, van t'Hoff's rule be applied to 
If all true catalyses are, as seems to be the case, diffusion phenomena, we 
should in fact expect in such reactions a uniformity of increment with rising tempera- 
ture much more marked than that noted by Euler. The variability shown by the 
