SECT. 2] AND WEIGHT 515 



2- 1 6. Temperature Characteristics 



The older phase of the subject ended, then, in a rather barren 

 doubt as to whether the van't Hoff equation was apphcable to hving 

 processes such as those of the growing embryo, or more correctly 

 whether the time/temperature relation was best expressed by a curve 

 of exponential form or by a hyperbola. In so far as it was applicable, 

 it gave definite information that the limiting factor of embryonic 

 growth was probably chemical rather than physical, but that was 

 all. Snyder's paper of 1908 introduced a new period, that of the 

 use of the Arrhenius equation. This expression is 



K^ = K^eHk'i) or ^ = eHk-fX 



where K^ and Kj^ are the velocity constants of the reaction in question 

 at the high and low temperatures, Tg and 7"i, chosen respectively, 

 e the base of Napierian logarithms, 2 the gas constant, and /a the 

 gram molecular energy of activation of the catalyst, i.e. the "critical 

 increment of the active substance" if the reaction is monomolecular, 

 or the sum of the gram molecular energies of the substances if the 

 reaction is bimolecular. The temperature is expressed in degrees 

 absolute. If the velocity constants cannot be calculated, the reci- 

 procals of the time taken to do a definite amount of embryo formation 

 or other work may be used instead, so that the relation becomes : 





T2- Ti = 10, then -^ = d^^ 



where ^^ and ^2 ^re the times in question, e.g. 10 days from fertilisa- 

 tion to hatching at one temperature, 20 days at another. When 



of the van't Hoflf equation. The relation may also be expressed: 



r log Ko — log K-, 



a = 4'6l —2 — 2 2 — i. 



II 



It was originally suggested by Arrhenius as an empirical description 

 of the facts, but it now has through the work of Rice; Rodebush; 



33-2 



