PHYSICAL FORCES 85 



powders will lie perfectly motionless when mixed 

 together in a dry state, but become violently turbulent, 

 when together thrown into a glass of water. Every 

 one knows that such changes are but transitory, as 

 well as that a telegraph wire, however much used, is 

 not perpetually active ; and some readers (who have 

 attended scientific lectures) may probably have learned 

 that a body may be magnetic or not magnetic, according 

 to circumstances. 



Thus a conception has very naturally arisen that 

 these manifestations of activity in material things 

 activities which we will speak of as physical forces 

 are entities or influences, which come and go which 

 pervade bodies for longer or shorter periods, and then 

 leave them as (to use a rough-and-ready illustration) 

 a sponge may be soaked full of water and squeezed dry 

 again, any number of times. 



The present chapter will be devoted to an exposition 

 of some elementary facts concerning these pervading 

 influences, the energies or "forces" to continue the use 

 of a popular term known as heat, light, sound, chemical 

 change, electricity, and magnetism. 



It is evident that bodies which every now and then 

 exhibit any one of these forces, continue to ^possess, at 

 times when they do not exhibit it, the power of again 

 manifesting it when the necessary conditions return. 

 The energy under such circumstances is said to be in a 

 " potential condition," as distinguished from an active, or 

 as it is technically termed, " kinetic," state. Such poten- 

 tial energy is a capacity for a certain activity e.g., doing 

 a certain amount of work which capacity is actively 

 expended in overcoming some definite resistance e.g., 

 overcoming it through a definite distance and is there- 

 fore capable of measurement. The term " force " denotes 



