168 LAWRENCE R. PROUTY AND JAMES D. HARDY 



ceding section. They can be modified to fit almost any biophysical 

 requirement ranging from quantitative studies of photosynthesis in 

 plants by highly sensitive, quick-responding thermocouples to deter- 

 mination of the rectal temperatures of an elephant. Chemical reac- 

 tion rates are affected to an even greater degree by temperature than 

 purely physical reactions such as conduction, convection, and dif- 

 fusion. Photochemical reactions are affected very little. Reactions 

 involving ionic exchange and neutralization of acid and alkali are so 

 extremely rapid that temperature changes are immeasurable. In 

 those reactions that proceed with measurable speed, the importance 

 of temperature received added emphasis by the publication of the 

 van't Hoff relationship (1884). It was shown that the velocity of a 

 chemical reaction is at least doubled by a 10°C. rise in temperature. 

 This discovery led to the use of the "temperature coefficient" (Qio) 

 to express the relation of the reaction velocity at a given temperature 

 to that at 10°C. lower. It became apparent that the van't Hoff rela- 

 tionship might be used to decide whether certain physiological proc- 

 esses such as the conduction of a nerve impulse had a physical or chemi- 

 cal basis. An attempt by Snyder (1908) to decide the nerve conduc- 

 tion problem was inconclusive because the Qio value of approximately 

 2 he obtained is borderline between physical and chemical reactions. 

 In determining temperature effects on the hydrolysis rate of su- 

 crose, Arrhenius (1889) (33) broadened the van't Hoff relationship by 

 expressing it as the differential equation : 



f/(log k)/dt - ^i/RT' (9) 



where k is the reaction velocity constant, R the gas constant (= 1.98 

 or roughly 2 cal.), and n'ls a term that Arrhenius at first thought had 

 no physical meaning but that was later shown experimentally to be a 

 constant of thermodynamic significance. The term n became known 

 as the "temperature characteristic" of a reaction. According to the 

 kinetic theory, the velocity of any chemical reaction is governed by 

 the number of effective molecular collisions in a given time. In 

 order for molecules to react, they must collide with relatively great 

 energy (energy of activation). The temperature characteristic, ii, of 

 any chemical reaction is defined as the energy required to "activate" 

 the molecules entering into the reaction and is expressed in calories 

 per gram molecular weight of reactant. 



According to Crozier (34), the temperature characteristic may 

 represent the critical increment of energy for the formation of active 



