66 THE PHILOSOPHY OF BIOLOGY 



engine we have (i) a mechanism of parts which do 

 not change in material and relationships to each other 

 (boiler, cylinder, pistons, cranks, slide-valves, etc.) ; 

 and (2) a working substance (the steam). 



Energy in the form of the chemical potential of coal 

 and oxygen is supplied to the mechanism. The coal 

 is oxidised, producing heat. The heat then expands 

 the working substance (the water in the boiler), and 

 this working substance — now a gas at high tempera- 

 ture and pressure — propels the piston and confers 

 kinetic energy on the engine. Note the essential steps 

 in this process : substances of high chemical potential 

 (coal and oxygen) suffer transformation into substances 

 of low chemical potential (carbon dioxide and water), 

 and the difference of energy appears as high-tempera- 

 ture heat (increased kinetic energy of water molecules, 

 to be more precise). This heat is then transformed 

 into mechanical work (the kinetic energy of the mole- 

 cules of steam is imparted to the piston of the engine). 

 But in this transformation only a relatively small 

 proportion (10% to 20%) of the energy available 

 is transformed into mechanical work : the rest is 

 dissipated as irrecoverable low-temperature heat, by 

 radiation from boiler, steam-pipes, engine, and as the 

 heat which passes into the condenser water. 



In the organism in general there is no distinction 

 between the fixed parts of the mechanism and the 

 working substance. The organism itself (its muscles, 

 nerves, glands, etc.) is the working substance. Further, 

 it is not quite certain that there is a necessary trans- 

 formation of chemical energy into heat. The source 

 of energy in the case of the warm-blooded animal is 

 the chemical energy of the food substances and oxygen 

 taken into its body. These chemical substances 

 undergo transformations in the alimentary canal and 



