THE PRINCIPLES OF ENERGY 45 



And by reversing statements (b) and (d) and (e) we get 



(/) 1 Calorie per second=5-62 h.p. 

 (g) 1 Calorie per second=4:-18 kilowatt 

 (h) 1 h.p. =0-746 kilowatt. 



But equations (e), (/), (g), and (h) are only true in theory, and 

 they are not true in practice. They would be if equations (b) and 

 (d) were reversible, which they are not. And that shows that we 

 must make a very clear distinction between available energy 

 (which is the capacity for doing work) and energy in the abstract, 

 which is understood when we disregard experience and consider 

 all transformations as reversible ones. 



Energy in the Abstract. 



When an energy transformation occurs, one kind of available 

 energy is converted into other kinds ; for instance, so many cubic 

 feet of coal gas are supplied to a gas engine, which then does 

 mechanical work. Now the " calorific value " of the gas can 

 easily be determined that is, we can find the quantity of heat 

 into which the chemical energy of the gas is completely trans- 

 formed when it is burned. So, also, the heat equivalent of the 

 work done by the engine could be determined; for instance, the 

 latter could be employed to lift a mass against the earth's gravity, 

 and then this work can be represented as heat by using the 

 mechanical equivalent in heat of this work done [equation (6), 

 p. 44]. There will be a balance, for the heat equivalent of the 

 gas is greater than the heat equivalent of the work done by the 

 engine, and so available energy is lost. But we know very well 

 that this available energy is really lost as heat radiated away 

 from the engine, lost in the products of combustion (which are 

 still hot when they are blown out into the air), and by other 

 " leakages." And so the energy is not really destroyed, but 

 simply dissipated. In many cases it can be traced, and where 

 it cannot be traced we assume that it might be. This leads us to 

 consider energy in the abstract without considering whether or 

 not it is available, or possesses the capacity for doing work. 



First of all, however, we must make the concept of an isolated 

 system. Let there be a large island which has absolutely no 

 commerce or other communications with the rest of the world. 

 It produces all that its inhabitants require, and it utilises itself 

 all that it produces. It is self-sufficient, and is economically an 



