ix WORK AND ENERGY 123 



certain electrified bodies also, and when such bodies are attracted 

 as a result of this electrification we see that electrification must 

 similarly be looked upon as still another kind of energy. But it 

 must be borne in mind that electrification is not electricity. 

 Then, too, there is the attraction of magnetism, which is capable 

 of accomplishing work, and hence must likewise be looked upon 

 as a form of energy. Chemical combinations, again, are always 

 accompanied by the development of heat, and resulting as they 

 do from the chemical attraction of two more or less unlike forms 

 of matter, we shall be right in saying chemical combination 

 are always accompanied by energy changes and so in regarding 

 chemical attraction as another kind of energy. In addition to 

 the energy of moving bodies we have energy manifested as 

 sound, heat, light, electrification, magnetism, and chemical 

 action. 



Bearing this in mind, it will perhaps assist the student to 

 grasp the enlarged conception of energy which is here presented 

 to him, if he regards energy as being a capacity for prodnchnj 

 physical change. 



Heat as a Form of Energy. Heat was not always regarded 

 in this way. It was originally thought to be a fluid called 

 Caloric, and it was supposed that a piece of hot iron differed 

 from a cold piece in having entered into some sort of union with 

 this fluid. But, since the experiments of Rumford, we can no 

 longer doubt that heat is not material, but a form of energy. 

 Rumford boiled water by the heat developed by the friction 

 between two metal surfaces which he rubbed together ; and he 

 found that the amount of water he could bring to the boiling 

 temperature depended only on the amount of work he expended 

 in rubbing. Since he could obtain an indefinite amount of 

 heat from two definite masses of metal, it was quite clear that 

 heat could not be matter, which, as we have seen, cannot be 

 created. Davy made the truth even clearer by obtaining heat 

 enough to melt ice by simply rubbing two pieces of this solid 

 together. They were both cold or without caloric ; and since 

 heat could be obtained by rubbing even these together, it was 

 quite certain that heat could not be a fluid. Joule went a step 

 further and measured the amount of work which must be done 

 to obtain a given quantity of heat ; or, as we say, he measured 

 the mechanical equivalent of heat. 



Some examples which will be familiar to the student will 



