672 TEIVIPERATURE AND LATENT PERIOD 



time sufficiently large to be measured comfortably. In addition, the 

 sensitization perijd is quite small, averaging 0.21 second for the 

 temperatures used. The actual manipulation is simple. An animal 

 which has been in the dark for a number of hours is brought to the 

 desired temperature using the ordinary precautions of stirring, etc. 

 With the temperature constant, the reaction time is determined five 

 times at 5 minute intervals. Readings are made approximately 5° 

 apart, 20 minutes being allowed for the acquisition of the desired 

 temperature. 



The results which were obtained in six experiments are given in 

 Fig. 2. Each point is an average of the five readings made at that 

 temperature. The experiments were arranged to obtain a uniform 

 distribution of determinations covering the range of temperatures to 

 be investigated. In general the data are similar to those which have 

 already been pubHshed. Considered in the usual way, it may be 

 said that the temperature coefficient is approximately 2.5, being 

 larger at lower temperatures and smaller at higher temperatures. 



The data, however, possess much more meaning than this. Be- 

 cause of our knowledge of the chemical reactions underlying the 

 reaction time, it is possible to make a quantitative analysis of the 

 results. Such an analysis is significant not only of itself, but 

 because it demonstrates the possibility of the quantitative treatment 

 of biological data of this character. 



III. 



Arrhenius has long ago shown that the now biologically famous 

 van't Hoff rule is not an exact statement of the facts (Arrhenius, 1912, 

 p. 124). Moreover, even as an approximation it lacks the theoretical 

 significance which is possessed by van't Hoff's equation relating the 

 temperature and the equilibrium constant of a chemical reaction. 

 Arrhenius has therefore derived, as a special case of the latter rela- 

 tion, an equation which relates the velocity constant of a chemical 

 reaction with the temperature. If k' and k" are the velocity constants 

 at the absolute temperatures T' and T" , this equation of Arrhenius 

 states that 



k" R\T' T") ,,. 



— = e (Jl 



k' 



