UFE AND ENERGY ii 



be of various kinds. Let us consider for a moment what sorts of 

 properties objects may possess in virtue of which we can get them 

 to do work for us. A bullet is a very different thing lying on the 

 table from what it is just after it has left the rifle barrel. In the 

 latter case, it can do work because it is moving ; it is said to 

 possess " kinetic energy." A reservoir of water at a high level can 

 do work as it falls to a lower level, as through the turbine or over 

 the mill-wheel. This is due to gravity. If the water remains 

 dammed up, the energy is there, but not in use ; we say that 

 it is potential. The fire under a steam boiler makes the engine 

 do work we have heat energy. The current of electricity in an 

 electro-motor enables it to drive machinery. The rays we receive 

 from the sun, some of which we call light ^ do an immense amount 

 of work. We may call this radiant energy. One of the most 

 important sources has not yet been mentioned. That is, .the 

 energy of chemical combination. Certain chemical substances, 

 when they combine together, give off energy in various forms. 

 These substances must originally have contained it in a potential 

 form. Consider the petrol of an internal combustion engine. It 

 gives off energy when it combines with the oxygen of the air. 

 The products, carbon dioxide and water, contain no energy that is 

 available for use. The greater part of the chemical energy that 

 we meet with is derived from combination with oxygen, which we 

 call combustion. The energy of our own bodies has the same 

 origin ; we burn up our food by means of oxygen obtained from 

 the air. The fact that one form of energy can be converted into 

 others is very obvious in this case. In the steam engine, the 

 combustion of the fuel comes out as kinetic energy. /.IT the engine 

 drives a dynamo, the energy of the combustion appears in part in 

 an electrical form. We obtain heat from our house fires, and light 

 from the burning of candles or from the electrical current. And 

 we can convert our electrical current back again into chemical 

 energy by the decomposition of water or by the use of the storage 

 battery. 



T\\t first law of energetics is the expression of the fact found 

 to be true whenever it is tested, namely, that any form of energy 

 can be converted into any other form of energy, and that there is 

 no loss and no gain in the process. This is always found to be 

 true. If we measure accurately the amount of energy, supplied to 

 a motor by the current, and also that which is given out by it as 

 mechanical work, together with that appearing as heat in the 

 motor itself and the other parts of the arrangement, we find them 

 exactly equal. Similarly, if we compare the chemical energy of 

 the fuel burnt in a petrol motor with the mechanical work done 

 and the heat produced, we find them equal. 



