INCOME AND EXPENDITURE OF THE BODY. 303 



form but that its amount is invariable, that it cannot. V>P nrpafpd 

 or destroyed but simply transmuted, is known as the law of 

 tlie Uonservaiion of Energy (see Physics), and, like the inde- 

 structibility of matter, lies at the basis of all scientific con- 

 ceptions of the universe, whether concerned with animate or 

 inanimate objects. 



Since all forms of energy are interconvertible it is con- 

 venient in comparing amounts of different kinds to express 

 them in terms of some one kind, by saying how much of that 

 standard form the given amount of the kind spoken of would 

 give rise to if completely converted into it. Since the most 

 easily measured form of energy is mechanical work this is 

 commonly taken as the standard form, and the quantities of 

 others are expressed by saying how great a distance against 

 the force of gravity at the earth's surface a given weight could 

 be raised by the energy in question, if it were all spent in 

 lifting the weight. The units of mechanical work being the 

 kilogrammeter or the foot-pound, the mechanical equivalent 

 of any given kind of energy is the number of kilogrammeters 

 or foot-pounds of work its unit quantity would perform if 

 converted into mechanical work and used to raise a weight. 

 FW example the Uhlt quantity oi heaTTs that necessary to 

 raise one kilogram of water one degree centigrade in temper- 

 ature; or sometimes, in books written in English, the quan- 

 tity necessary to warm one pound of water one degree Fahren- 

 heit. When therefore we say that the mechanical equivalent 

 of heat is 423 kilogrammeters we mean that the quantity of 

 heat which would raise one kilogram of water in temperature 

 from 4 C. to 5 C. would, if all turned into mechanical work, 

 be able to raise one kilogram 423 meters against the attraction 

 of the earth; and conversely, that this amount of mechanical 

 work if turned into heat would warm a kilogram of water 

 one degree centigrade. The mechanical equivalent of heat, 

 taking the Fahrenheit thermometric scale and using feet and 

 pounds as measures, is 772 foot-pounds. 



Potential and Kinetic Energy. At times energy seems 

 to be lost. Ordinarily we only observe it when it is doing 

 work and producing some change in matter:, but sometimes 

 it is at rest, stored away and producing no changes that we 

 recognize and thus seems to have been destroyed. Energy at 

 work is known as kinetic energy; energy at rest, not produc- 

 ing changes in matter, is called potential energy. Suppose a 



