ENERGY. ii 



There are several forms of energy (heat, motion, electricity, 

 and attraction, for instance), all of which can mutually repro- 

 duce each other. We may divide attraction into gravi- 

 tation, cohesion and chemical affinity. Gravitation is the 

 power of attraction which all bodies in the universe have for 

 each other. Cohesion and chemical affinity act only when the 

 bodies (molecules or atoms) which are influenced by them, are 

 very close together. Electricity, as we all know, can generate 

 motion, light and heat. Heat by means of the steam engine, 

 for example, can produce motion and electricity. Motion 

 produces heat when we rub the palms of our hands briskly 

 together ; electricity and heat, when we rub a stick of sealing 

 wax on a silk handkerchief; and light and heat, when, by 

 using a flint and steel, the small particles of stone which are 

 broken off, violently impinge against each other, and thus 

 produce sparks. The principle here involved is termed the 

 correlation of forces, and means, as Grove expresses it, " that 

 any force capable of producing another may in its turn be 

 produced by it." 



Energy exists either as active energy ', which is energy at 

 work, or passive energy (potential energy), which is energy at 

 rest. If we wind up a clock without the works being put into 

 movement, the active energy in the form of motion which we 

 transferred from our muscles to the machinery, will remain as 

 passive energy until the pendulum begins to swing. Then, 

 as long as the works keep going, their motion will become 

 converted into heat. If we throw a stone straight up into the 

 air, the active energy possessed by the stone on quitting our 

 hand, will gradually become converted into passive energy by 

 the force of gravitation, until the highest point of its trajec- 

 tory is reached, at which moment it will be at rest. During 

 its descent, it will gradually lose its accumulated passive 

 energy, until on striking the ground, its active energy of 

 motion, due to gravitation, will become converted into heat. 



