(7) 



43 



using the term force in its widest sense may be divided 

 into attraction and motion, so all energy is divided into 

 potential and actual energy, synonymous with those 

 terms. It is the chemical attraction of the atoms, or 

 their potential energy, which makes gunpowder so pow- 

 erful ; it is the attraction or potential energy of gravita- 

 tion which gives the power to a raised weight. If now, 

 the impediments be removed, the power just now latent 

 becomes active, attraction is converted into motion, 

 potential into actual energy, and the desired effect is 

 accomplished. The energy of gunpowder or of a raised 

 weight is potential, is capable of acting ; that of explod- 

 ing gunpowder or of a falling weight is actual energy 

 or motion. By applying a match to the gunpowder, by 

 cutting the string which sustains the weight, we convert 

 potential into actual energy. By potential energy, there- 

 fore, is meant attraction ; and by actual energy, motion. 

 It is in the latter sense that we shall use the word force 

 in this lecture ; and we shall speak of the forces of 

 heat, light, electricity and mechanical motion, and of 

 the attractions of gravitation, cohesion, chemism. 



From what has now been said, it is obvious that when 

 we speak of the forces of heat, light, electricity or mo- 

 tion, we mean simply the different modes of motion 

 called by these names. And when we say that they 

 are correlated to each other, we mean simply that the 

 mode of motion called heat, light, electricity, is convert- 

 ible into any of the others, at pleasure. Correlation 

 therefore implies convertibility, and mutual dependence 

 and relationship. 



Having now defined the use of the term force, and 

 shown that forces are correlated which are convertible 



