Matter and Energy. 13 



Further, by the agency of, say, a steam engine, the weight 

 might be again raised to the top of the tower, i.e. the kinetic 

 energy of the engine may be retransformed into the potential 

 energy of the raised weight. 



Again, the weight at the tower's top is prevented from 

 falling by the resistance of the tower. It requires an exer- 

 tion of force to overcome this resistance. That counteract- 

 ing force implies a source of energy which must manifestly 

 be a form of kinetic energy. Imagine the weight referred 

 to as suspended from a hook the energy employed to 

 liberate the weight from its support must be a kinetic 

 energy. The liberating energy need not be by any means 

 commensurate to the effects produced by the liberation. 

 The old proverb, 'a small spark may raise a great con- 

 flagration,' accurately expresses this fact. Indeed, we may 

 take this proverb as an illustration. A barrel of gunpowder 

 consists of a mixture of a large number of molecules of 

 different substances which under certain conditions have a 

 very strong tendency to unite and form new combinations. 

 So long as the condition of their union be. absent, the 

 molecules retain energy of position, and the barrel of gun- 

 powder is a store of potential energy. The condition of 

 their union is the production of a high temperature in their 

 immediate vicinity. That condition being satisfied by the 

 application of a spark to the mass, the affinities of the mole- 

 cules become at once satisfied, and the store of potential 

 energy becomes transformed into kinetic energy, accom- 

 panied by the usual attendant phenomena of an explosion, 

 viz. sound, light, and heat. 



It is necessary that we should trace this conception of 

 energy a little farther. 



Kinetic energy being energy of motion, the various 

 modes of motion may be conversely termed kinetic energies. 

 Sound is produced by the vibration of molecules, and is 

 communicated to the organ of hearing by the sympathetic 

 vibration of the mplec r ul<es-e^ rtie atmosphere between the 



