THE PROBLEM OF FOOD 5 



there is difficulty in looking upon energy as a real 

 thing- is that it cannot be seen and handled as matter 



't- 



is. But one of the most significant and fundamental 

 discoveries of modern science is that there is no loss 

 and no creation of energy, although one form of it can 

 be converted into a corresponding amount of another 

 form. Of these various forms we may mention heat, 

 electricity, motion, energy of chemical change, and 

 so on. The most generally known case in which we 

 actually pay for units of energy as such is that of 

 the electrical energy which we use for heat and light. 

 The facts described above will be recognized by many 

 readers as the first law of energetics. 



Since, then, any form of energy can be converted 

 into any other, it is convenient to express them all, for 

 the purposes of measurement, in the same unit. And, 

 since all forms can be easily converted into heat, this 

 is chosen. The unit of heat energy is called "calorie". 

 One calorie is the amount of heat required to raise 

 the temperature of i kilogram of water by i C. 



In this process of conversion of one form of energy 

 into another, we come across the second law of ener- 

 getics. Although we can convert any other form of 

 energy completely into heat, we find that the reverse 

 process cannot be completely effected; there is always 

 some heat left unchanged. The proportion is given 

 us by the law mentioned. The fact itself is doubtless 

 connected with the circumstance that we are dealing 

 with heat energy a long way above its zero state. 



When we have done work, then, we have lost 

 energy, and, in order to be able to do further work, 

 we must be supplied with more energy. Whence do 

 we get this? It is in the following way : The materials 

 we take as food contain chemical energy which can 

 be converted into other forms heat, muscular motion, 

 and so on when they are combined with oxygen, or 



