12 Elementary Biology. 



SECTION II. ENERGY. 



Work is the production of motion in mass or molecule 

 against resistance. Work is done when a mass of matter 

 is lifted against the resistance of gravity ; work is done 

 when a chemical compound is decomposed, for the resistance 

 of the peculiar form of attraction known as chemical affinity 

 is overcome and the molecules are separated from each 

 other. 



Energy is the power to do work. It must be carefully 

 distinguished from force, the existence of which it, however, 

 implies, and from motion which, as being produced by force, 

 indicates the presence of energy. Thus, force is exerted 

 when a weight is lifted in virtue cf the arm of the labourer 

 possessing a store of energy, a certain proportion of which 

 energy is used in the act of lifting the weight. Force is 

 similarly exerted when a compound is decomposed in 

 virtue of the agent in the decomposition possessing a store 

 of energy, some of which is expended in the performing of 

 the work of decomposition. 



When a mass of matter rests on the earth's surface, it 

 exerts a certain force due to the action of gravity ; but it 

 possesses no energy, it possesses no motion of its own. 

 When the mass is raised, say to the top of a high tower, 

 the force of gravity has been, in accordance with the law 

 of gravity, reduced, by an extremely small but calculable 

 amount, because its distance from the centre of the attract- 

 ing body has been increased. It has no motion of its own, 

 but it has gained a certain amount of energy energy cf 

 position, or potential energy. It is capable of doing some 

 work ; it is capable, for instance, of driving a pile into the 

 ground if allowed to drop upon it. When the mass reaches 

 the ground again, it has lost its energy. Its energy in the 

 act of falling has become transformed into active or kinetic 

 energy. Kinetic energy is therefore energy of motion. 



