PHYSIOLOGY: S. HECHT 
147 
chemical reaction which is catalyzed by the substance formed during the ex- 
posure to light. 
This intimate connection between the two phases of the reaction time makes 
it possible to investigate the relation between the temperature and the latent 
period, free from the usual 'rate of heating' and 'time factor' difi&culties. By 
bringing the animal to the desired temperature in the dark, errors resulting 
from the change of the system during the preliminary and subsequent heat- 
ings are largely eliminated. This is because the products of the reaction con- 
cerned with the latent period occur only as an immediate result of the exposure 
to light. They can, therefore, be influenced only after such exposure. The 
changes which then occur are exactly the ones which it is desired to measure. 
The results obtained by determining the latent period at different tempera- 
tures, at a constant intensity, will be found in detail in a future number of the 
Journal of General Physiology. They are, in general, similar to the many 
that have already been published, with the exception that they are amenable 
to a clear analysis. With this we shall now be concerned. 
3. The relation between the velocity constants {Ki and Kq) of a chemical 
reaction at different temperatures (Ti and To), is given by Arrhenius in the 
following equation: 
The temperatures are absolute, e is the Naperian base, and /x is a constant 
characteristic of a given chemical reaction. Since only the ratio between the 
velocity constants is required, there may be substituted in their places the 
reciprocals of the time required to accomplish a given amount of work. 
Using the reciprocals of the latent period at different temperatures in this 
manner, /x shows a constant value of 19,680 from 13° to 21°C. Between these 
temperatures the latent period, therefore, varies as if it were conditioned by a 
single chemical reaction. Above 21°, however, the value of jj, decreases 
steadily until at 31° it is 11,210. This regular decline in the value of jj, is 
equivalent to saying that above 21° the latent period becomes longer than it 
would be if the process as a whole showed a constant value of ;u. At higher 
temperatures, therefore, the operation of a secondary factor becomes evident. 
The effect of this is to increase the duration of the reaction concerned funda- 
mentally with the latent period. 
4. The steady decrease in the value of fj, with rising temperature indicates 
an increasing effect of the second factor. The results of its operation below 
21° are probably not measurable. It should be pointed out that 21°C. is the 
mean water temperature of the hottest month at Woods Hole. 
If we conceive the substance formed during the latent period to be ther- 
molabile, and also that a definite quantity of it is required to cause a response 
of the animal, the presence of this secondary, modifying factor may be quan- 
titatively determined. At higher temperatures the thermolabile substance is 
