( 605 ) 
Here we wish still emphatically to point out that it has never been 
our intention to underrate the great signification of the introduction 
of the conception “limiting factors” by BrACKMAN; what by this 
investigator has in general been observed about this point, preserves 
its full vigour, also when accepting our conclusions. 
A third objection which Mr. Rureers brings forwards against our 
views as to the course of the temperature curve beneath the optimum, 
is that also at various common chemical reactions quite analogous 
deviations from the rule of van ’r Horr have been stated. We think, 
however, that this well-known fact must not be advanced against 
our conclusion. For, admitting that for physiological processes, also 
at harmless temperatures, the rule of vaN ’T Horr is of no consequence 
what then remains of BrACKMAN’s theory ? 
For in this case there is not the least objection to suppose that 
also in absence of noxious influences the value of the temperature 
coefficient lowers to 0 or even becomes negative. Even some physico- 
chemical processes are known which proceed with a decided tempe- 
rature optimum. So, the number of crystal germs produced at different 
temperatures in undercooled solutions, as TAMMANN’s ') researches 
prove. The most remarkable results in this respect are those of 
C. Ernst, whereby the relation between the velocity of the catalysis 
of hydro-oxygen by colloidal platinum is described. For this catalytic 
process, which exhibits in many respects interesting analogies with 
physiological processes, Ernst comes to the result that beneath a 
certain temperature, for different times of fore-heating one single curve 
is found for the relation: reaction-velocity-temperature, whilst besides, 
with different fore-heating times, also different curves are found. 
He further shows that fore-heating at higher temperatures causes 
a (lasting) noxious influence on the reaction velocity, which injury 
he also determines after the method followed by us. When now he 
calculates the curve: velocity-temperature for a fore-heating time 0, 
he also obtains a curve showing an optimum. The results obtained 
by this anorganic process are thus quite in accordance with those 
found by our experiments. 
It must still be remarked that during the time taken by his experi- 
ments performed to observe the temperature optimum (with fore- 
heating periods of 4 and 5 hours), Ernst did not detect a favourable 
influence by continued fore-heating. A slight decrease of velocity, hardly 
perceptible after 4 days, but after 14 days somewhat visible, found 
1) Zeitschr. f. Phys. Chem. Bd. 25, 1898, S. 441. 
