168 PRINCIPLES OF PHYSIOLOGY 



to the amount of energy he can liberate as 

 mechanical work and the ratio of this amount 

 to the amount of energy appearing as heat. 

 The best results show that about 25 per cent, 

 of the available energy appears as work, with 

 75 as heat. This compares favourably with 

 many human contrivances. The best steam- 

 engine can only give about 12 J per cent, of 

 the energy produced by complete combustion 

 of the fuel, but gas or petrol engines can do 

 much better as much as 96-98 per cent, of 

 energy can be transmuted by certain electrical 

 contrivances. The heat of the engine, how- 

 ever, is a real loss of energy, and all engineers 

 strive to reduce it to a minimum, but the heat 

 of the body is one of the all-important conditions 

 of its mechanism. The source of error in all 

 such estimations is that food stuffs are never 

 completely oxidised in the body, and further 

 that the methods followed by the engineer in 

 measuring the output of energy are different 

 from those of the physiologist. Still the 

 general statement is fairly correct. 



91. In some animals energy is liberated in the 

 form of electrical energy or light, as in the electric 



