94 THB HUMAN BODY. 



ent, kinds of matter are not. We cannot, as the alchemists 

 hoped, turn iron or mercury into gold, but we can turn 

 light into heat, and heat into electrical force, or into me- 

 chanical work. When such transformations are made it is 

 always found that a definite amount of one kind of energy 

 disappears to give rise to a certain definite amount of an- 

 other. In other words, it has been discovered that energy 

 cannot be created : if we take a given quantity of heat we 

 can turn some of it into mechanical work ; if we then, 

 turn all this mechanical work back into heat we get 

 again exactly the quantity of heat which disappeared 

 when the mechanical work appeared : and so with all other 

 transformations of energy from one kind to another, and 

 back again. This fact that energy or work-poiver can be 

 turned from one kind into another, and often lack again, 

 but never created from nothing or finally destroyed) is 

 known as the law of the conservation of energy. 



Illustrations of the conservation of energy. In a steam- 

 engine, heat, which is the best known kind of energy, is 

 produced in the furnace ; when the engine is at work all 

 of this energy does not leave it as heat ; some is turned into 

 mechanical work, and the more work the engine does the 

 greater is the difference between the heat generated in the 

 furnace and that leaving the machine. If, however, we 

 used the work to rub two rough surfaces together we 

 could get the heat back, and if (which of course is im- 

 possible in practice) we could avoid all friction between 



Can matter be transmuted ? What is always found when energy 

 is transformed ? 



Can man create energy ? Illustrate the fact that energy can be 

 changed in kind but not created. What is meant by the law of the 

 conservation of energy ? 



Give an illustration of the conservation of energy. 



