Ill] THE TEMPERATURE COEFFICIENT 221 



temperature to 1° C. ! Of two allied species, one ran nearly half as 

 fast again as the other, at the same temperature*. 



While at low temperatures growth is arrested and at temperatures 

 unduly high hfe itself becomes impossible, we have now seen that 

 within the range of more or less congenial temperatures growth 

 proceeds the faster the higher the temperature. The same is true 

 of the ordinary reactions of chemistry, and here Van't Hoff and 

 Arrheniusf have shewn that a definite increase in the velocity of 

 the reaction follows a definite increase of temperature, according to 

 an exponential law: such that, for an interval of n degrees the 

 velocity varies as x", x being called the "temperature coefficient" 

 for the reaction in question J. The law holds good throughout a 

 considerable range, but is departed from when we pass beyond 

 certain normal limits ; moreover, the value of the coefficient is found 

 to keep to a certain order of magnitude — somewhere about 2 for 

 a temperature-interval of 10° C. — which means to say that, the 

 velocity of the reaction is just about doubled, more or less, for a 

 rise of 10° C. 



This law, which has become a fundamental principle of chemical 

 mechanics, is applicable (with certain qualifications) to the pheno- 

 mena of vital chemistry, as Van't Hoff himself was the first to declare ; 

 and it follows that, on much the same fines, one may speak of a 

 "temperature coefficient" of growth. At the same time we must 

 remember that there is a very important difference (though we need 

 not call it a fundamental one) between the purely physical and the 



* Harlow Shapley, On the thermokinetics of Dolichoderine ants, Proc. Nat. 

 Acad. Sci. x, pp. 436-439, 1924. 



t Van't HofF and Cohen, Studien zur chemischen Dynamik, 1896; Sv. Arrhenius, 

 Ztschr. f. phys. Chemie, iv, p. 226. 



X For various instances of a temperature coefficient in physiological processes, 

 see (e.g.) Cohen, Physical Chemistry f or ... Biologists (Enghsh edition), 1903; 

 Kanitz and Herzog in Zeitschr. f. Elektrochemie, xi, 1905; F. F. Blackman, Ann. 

 Bot. XIX, p. 281, 1905; K. Peter, Arch.f. Entw. Mech. xx, p. 130, 1905; Arrhenius, 

 Ergebn. d. Physiol, vii, p. 480, 1908, and Quantitative Laws in Biological Chemistry, 

 1915; Krogh in Zeitschr. f.allgem. Physiologie,xyi, -pp. 163,178,1914; James Gray, 

 Proc. E.S. (B), xcv, pp. 6-15, 1923; W. J. Crozier, many papers in Journ. Gen. 

 Physiol. 1924; J. Belehradek, in Biol. Reviews, v, pp. 1-29, 1930. On the general 

 subject, see E. Janisch, Temperaturabhangigkeit biologischer Vorgange und ihrer 

 kurvenmassige Analyse, Pfluger's Archiv, ccix, p. 414, 1925; G. and P. Hertwig, 

 Regulation von Wachstum . . . durch Umweltsfaktoren, in Hdb. d. normal, u. pathol. 

 Physiologie, xvi, 1930. 



