THE CONSERVATION OF ENERGY. 281 



we could avoid all friction in the moving parts of the 

 machine, the quantity thus restored would be exactly equal 

 to the excess of the heat generated in the furnace over that 

 leaving the engine. Having turned some of the heat into 

 mechanical work we could thus turn the work back into 

 heat again, and find it yield exactly the amount which 

 seemed lost. Or we might use the engine to drive an elec- 

 tro-magnetic machine and so turn part of the heat liber- 

 ated in its furnace first into mechanical work and this into 

 electricity; and if we chose to use the latter with the 

 proper apparatus, we could turn more or less of it into 

 light, and so have a great part of the energy which first 

 became conspicuous as heat in the engine furnace, now 

 manifested in the form of light at some distant point. In 

 fact, starting with a given quantity of one kind of energy, 

 we may by proper contrivances turn all or some of it into 

 one or more other forms; and if we collected all the final 

 forms and retransformed them into the first, we should 

 have exactly the amount of it which had disappeared when 

 the other kinds appeared. This law, that energy can 

 change its form but that its amount is invariable, that it 

 cannot be created or destroyed but simply transmuted, is 

 known as the law of the Conservation of Energy (see Phy- 

 sics), and, like the indestructibility of matter, lies at the 

 basis of all scientific conceptions 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 sur- 

 face a given weight could be raised by the energy in ques- 

 tion, 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 



