1. The critical thermal increments are calculated for respiratory processes (O2 consumption, CO2 production) in various plants and animals. They are characteristically found to be of two, possibly three, types: µ = 11,500, and 16,100 or 16,700. The first is commonly encountered above 15°, the second below that temperature, but these relations may be reversed. (The value of µ may be significantly changed in inanition.)
2. For reduction of methylene blue by bacteria, through removal of H from succinic acid, µ = 16,700. This process (Quastel and Whetham, 1924) at constant temperature is a function of the hydroxyl ion concentration. The suggestive identity is pointed out of the critical increment for this reduction phenomenon with that deduced for biological respirations in which a dehydrogenation mechanism is supposed to be of widespread occurrence, and in connection with which Fe very likely has a catalytic rôle. The action of OH' is believed to be revealed in the value µ = 11,500, frequently obtained in connection with respiration.
3. A somewhat lower µ (16,140) is associated with the oxidation of Fe'', and may be compared with (1) that of respiration in sea urchin eggs, for which (Warburg) iron is catalyst, and (2) that for some simple reactions in which Fe is known to serve as catalyst; it is not found for oxidative reactions in which Fe is not involved.
4. The bearing of these findings is discussed in relation to the theory of functional analysis of concurrent catalyzed reactions in protoplasm. It is shown that for a number of activities in which the effects of respiration may safely be assumed, the values of the critical increments are consistent with those determined for processes of respiration.
5. The further development of these views may lead to an extremely important method of identifying controlling reactions in undisturbed living matter.