FOOD AS A SOURCE OF ENERGY 29 



in other words, though performing no work at the 

 moment, it is capable of doing so. For example, 

 if it be attached to a cord and the cord be put in 

 connection with a clock mechanism, the weight, if 

 swung free, is capable of making the clock go. Its 

 potential energy now becomes kinetic or active. 

 Physicists also tell us that energy may be recognised 

 in several different forms, such as chemical energy, 

 thermal energy, photic or light energy, electrical 

 energy, &c. By chemical energy, for instance, is 

 meant that form of energy which is exhibited when 

 constituent units or atoms combine to form a molecule 

 of a compound. Thus a molecule of water is re- 

 presented by the formula H.,O, meaning that water 

 is a compound of the two elements, hydrogen and 

 oxygen, in the proportion of two atoms of hydrogen 

 to one of oxygen. When these two gases are brought 

 together, under certain external conditions, they 

 combine with each other, and, in the act of uniting, 

 energy is set free. 



Then again, the results of physical research have 

 led to the conclusion that all these forms of energy are 

 reducible to one, and that each may be changed, 

 and, in Nature, is constantly being changed, into conserva- 

 another. Still, no matter how or to what extent energy, 

 the change occurs, the sum of the energies in the 

 Universe (if finite) is a constant quantity. It 

 cannot be reduced and it cannot be increased, it 

 can only be altered in form or in state. This is 

 known as the Law of the Conservation of Energy. 



In order to obtain exact data as to the amount 

 of energy expended we must fix on a standard 

 by which to measure energy. Manifestly this Measure- 

 must be relative only and in terms of one or 

 other of the various forms of energy. But which 



