ORGANISM AND MECHANISM 113 



it is important to notice that the working out of the chemico- 

 physical description of vital activity is not altogether plain 

 sailing. Let us illustrate. 



§ 3. Some Difficulties m the Application of Physical and 

 Chemical Formulw to Organisms. 



It is a general fact of experience that the rate of chemical 

 reactions is accelerated by heat and retarded by cold. The 

 illustrious chemist Van't Hoff formulated the law that the 

 rate of a chemical process increases in geometric progres- 

 sion when the temperature is increased in algebraic progres- 

 sion. The velocity of the reaction may be doubled or trebled 

 by a rise in temperature of 10° C. or reduced by one-half 

 or more by a fall in temperature of 10 ° C. Now it has been 

 observed that the rate of heart-beat of various animals, so 

 widely separated as tortoise and water-flea, is reduced to 

 about a half if the temperature be lowered 10° C, and the 

 same holds of some other vital processes. Therefore it has 

 been hastily concluded that the chemical processes associ- 

 ated with vital activities follow Van't Hoff's law in the way 

 they vary in rate with changes in temperature. But it 

 looks as if the conclusion had been premature. The increase 

 in the rate of development of the eggs of the plaice is 

 directly proportional to the increase in the temperature 

 within the limits of viability (Dannevig, Johansen, Krogh) ; 

 it does not illustrate Van't Hoff's law. In certain fishes, in 

 frogs, water-beetles, and sea-urchins the Danish physiologist 

 Krogh finds that the relation between the temperature and 

 the rate of development cannot be expressed, even approxi- 

 mately, by Van't Hoff's formula. According to Krogh's ex- 

 periments on frogs and goldfishes and some other animals, 

 the influence of temperature on the ' standard ' metabolism, 



