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PHYSIOLOGY: S. HECHT 
AN ANALYSIS OF THE RELATION BETWEEN THE 
TEMPERATURE AND THE DURATION 
OF A BIOLOGICAL PROCESS 
By Selig Hecht 
Department of Biochemistry and Physiology, College of Medicine, Creighton 
University, Omaha 
Communicated by J. Loeb, February 17, 1919 
1. Biological processes exhibit variations in rate at different temperatures ► 
The speed of a given activity increases with an increase in the temperature, 
usually in the form of an exponential function, occasionally in a linear manner. 
At higher temperatures, however, the rate of increase falls off. The velocity 
of the process reaches a maximum at a critical temperature, above which the 
activity declines rapidly or ceases entirely. 
Such vital processes are undoubtedly conditioned by chemical and physical 
reactions. It should therefore be possible to analyze these variations in terms 
of the changes usually associated with the relation between chemical and phys- 
ical reactions and the temperatures at which they occur. This has been dif- 
ficult to accomplish except in the most general terms. The variations at 
higher temperatures are attributed to the destructive effect of heat on proto- 
plasm, etc., but no really quantitative analysis of such variations is attempted. 
It has, however, been possible to accomplish this in a study of the photic sen- 
sitivity of My a arenaria. 
2. The mollusc My a responds to illumination by a rapid retraction of its 
siphons. Its reaction time is composed of two parts. The first is a sensiti- 
zation period during which the animal must be exposed to light in order to 
respond at the end of the usual reaction time. The second is a latent period 
during which Mya may remain in the dark. At the end of this period the or- 
ganism responds as if it had been exposed to light for the entire reaction time. 
The composition of the reaction time is strikingly clear, because the sensitization 
period is very short, — less than 0.10 second at moderate intensities. The 
latent period, however, is well above 1.0 second at room temperature. 
The sensitization period is conditioned by the velocity of a photochemical 
reaction. The more intense the stimulating light, the shorter is the time 
necessary to expose Mya in order to produce a response. 
The latent period, however, depends on a reaction, the velocity of which 
is determined by the amount of substance formed during the exposure to light. 
The reciprocal of the latent period measures the velocity of its determining 
reaction. The relation between the velocity and the exposure time is strictly 
linear for exposures shorter than the sensitization period. Since it is gener- 
ally true that the velocity of a catalyzed reaction is a linear function of the 
concentration of catalyst, the latent period is very probably the result of a 
