IS THE BODY A MACHINE? 21 



Energy, as already noticed, is the power of doing 

 work, and its ordinary active forms are heat, mo- 

 tion, electricity, light, etc. ; but it may be in a 

 passive or potential form, and in this form stored 

 within a chemical molecule. These various forms 

 of energy are readily convertible into each other ; 

 and any form of apparatus designed for the pur- 

 pose of producing such a conversion is called a 

 machine. A dynamo is thus a machine so adjusted 

 that whem mechanical motion is supplied to it 

 the energy of motion is converted into electricity ; 

 while an electromotor, on the other hand, is a 

 piece of apparatus so designed that when electric- 

 ity is applied to it, it is converted into motion. 

 A steam engine, again, is designed to convert 

 potential or passive energy into active energy. 

 Potential energy in the form of chemical compo- 

 sition (coal) is supplied to the engine, and .this 

 energy is first liberated in the active form of heat 

 and then is converted into the motion of the great 

 fly-wheel. In all these cases there is no energy or 

 power created, for the machine must be always 

 supplied with an amount of energy equal to that 

 which it gives back in another form. Indeed, a 

 larger amount of energy must be furnished the 

 machine than is expected back, for there is always 

 an actual loss of available energy. In the process 

 of the conversion of one form of energy into an- 

 other some of the energy, from friction or other 

 cause, takes the form of heat, and is then radiated 

 into space beyond our reach. It is, of course, not 

 destroyed, for energy cannot be destroyed ; but it 

 has assumed a form called radiant heat, which 

 is not available for our uses. A machine thus 

 neither creates nor destroys energy. It receives 

 it in one form and gives it back in another form, 



