38 THE MECHANISM OF LIFE 



A dynamo will remain at rest and yield no current unless work 

 is done upon it, and this also we measure in h.p. Supply motive 

 power equal to so many h.p. to the dynamo, and the latter, with 

 its conductors, changes its state and develops so many kilowatts 

 of current. 



Finally, a man may climb a mountain, say 5,000 feet high, in 

 six hours, but not unless certain substances in his body become 

 oxidised, yielding muscular power capable of raising his body 

 against the resistance of the earth's gravity. But, again, he 

 cannot supply this muscular power and do work unless he takes 

 food, and a certain quantity of the latter — that is, so much 

 proteid, fat, and carbohydrate — estimated in equivalent heat 

 units, called Calories, must be supplied before his body can do 

 the specified work. 



In all these cases something called the capacity for doing work 

 is added to the thing that changes or does work. Chemical sub- 

 stances (the fuel and oxygen) are burned in the steam boiler, 

 electric current is fed into the motor, mechanical motion is im- 

 parted to the dynamo, and chemical substances are assimilated 

 into the muscles of the mountaineer. All these things represent 

 ,the capacity for doing work, and the latter we define more shortly 

 as available energy. 



Obviously there are different forms of available energy, and 

 these can be transformed one into the other. There is mechanical 

 energy, that of the motions of material bodies — for instance, the 

 motion of the parts of the locomotive engine; heat energy — that 

 is, the enormously increased velocity of movement of the mole- 

 cules of something or other; elecjiiric energy, which is the flow of 

 electrons through a conductor; mugcular energy (about which we 

 know very little) ; gravitational energy ; the energy of radiation, 

 etc. We know little or nothing of what these various forms of 

 available energy are, although we recognise them as different, for 

 they affect our sense organs differently; thus we recognise 

 mechanical energy because we exert muscular power in opposing 

 it; we recognise heat through the stimuli of certain sense organs; 

 radiation usually by the stimuli of the retina (and also of the 

 skin); electric energy mainly by its stimulation of the muscles, 

 and so on. 



But all forms of available energy can be converted one into the 



^ other, and all of them are the capacity for doing work, or more 



generally the state of something or other. But, again, in what- 



