THE ORGANISM AS A MECHANISM 69 



ing work. In the heat-engine chemical energy trans- 

 forms to heat, and then to mechanical energy, and of 

 the total quantity transformed a certain large pro- 

 portion suffers dissipation by conversion into low- 

 temperature heat. In the animal organism chemical 

 energy transforms directly to mechanical energy with- 

 out passing through the phase of heat. If heat is 

 produced it is because it is, in a way, available energy, 

 inasmuch as it permits of the continuance of chemical 

 reactions at a normal rate. The analogy of the animal 

 with the heat-engine is therefore a false one. It 

 suggests oxidation of the food-stuffs and heat produc- 

 tion, whereas it is not at all certain that any significant 

 proportion of the energy of the organism is the result 

 of oxidation : many animal organisms indeed function 

 in the entire absence of free oxygen. Further, the 

 proportion of energy dissipated is always small com- 

 pared with the heat-engine, and tends to vanish. The 

 second law of thermodynamics does not, then, restrict 

 the energy-transformations of the animal organism to 

 the same extent that it restricts the energy-transform- 

 ations of the physico-chemical mechanism. 



The processes involved in the plant organism 

 differ still more in their direction from those of a 

 " purely physical " train. To see this clearly we must 

 consider the imaginary mechanism known as a Carnot 

 heat-engine. 1 This is a system in which we have (i) a 

 heat-reservoir at a constant high temperature, (2) a 

 refrigerator at a constant low temperature, and (3) a 

 working substance which is a gas. Energy is drawn 

 from the reservoir in the form of heat, and this heat 

 expands the gas, doing work. The gas contracts, and 

 its heat is then given up to the refrigerator. The 

 work done is equal to the difference between the amount 



1 See appendix, p. 363. 



