148 
PHYSIOLOGY: S. HECHT 
rendered inactive in appreciable amounts. Consequently the reaction pro- 
ducing the thermolabile substance must proceed longer in order to make up 
the amount of it required for a response. The interval of time in which all 
this occurs is very small at temperatures above 21°, — less than one second. 
The portion of the reaction isotherm which is involved may therefore be con- 
sidered a straight Hne. The amoimt of thermolabile substance formed will 
thus be a linear function of the time. 
By using the original, constant value of ( = 19,680), it is possible to cal- 
culate the duration of the latent period at higher temperatures, free from the 
influence of the additional variable. The differences between the calculated 
time and the observed time serve as a measure of the effect of the second 
factor. The difference in time divided by the calculated time gives the per- 
cent of the theoretically necessary amount of thermolabile substance which is 
inactivated at any temperature. 
It has been repeatedly shown that the course of inactivation reactions and 
of spontaneous decompositions agrees well with that of a reaction of the first 
order. Therefore, from the above results, isotherms may be constructed 
showing the partial course of the inactivating reaction here concerned, at 
different temperatures. It is thus possible to find the time required to inac- 
tivate a given amount of thermolabile substance at the various temperatures. 
The reciprocals of the time may now be substituted in the above equation of 
Arrhenius, and the value of /z determined. 
When these steps are followed, the inactivating reaction shows a constant 
value of ju = 66,800. High values for /x, of the same order of magnitude as 
here given, obtain generally for spontaneous destructions, inactivations, and 
coagulations. The fact that the value of n corresponds to those found for 
similar processes strengthens the force of the reasoning which underlies the 
analysis here presented. 
It is, therefore, permissible to conclude that the duration of the latent 
period in the photic response of Mya, at the temperatures studied, is con- 
ditioned by two chemical reactions. The first is the fundamental reaction 
which determines the nature of the latent period. The second is a reaction 
which inactivates the principal product of the fimdamental reaction, thereby 
prolonging the latent period at higher temperatures. 
