these definitions, in fact, appear to comprise both force and 

 en erg}' . 



8. The definition of force which appears to the writer least 

 open to objection, is — that which produces a mutual action 



BETWEEN DIFFERENT PORTIONS OR PARTICLES OP MATTER, BY 

 WHICH THEY ARE EITHER ATTRACTED TOWARDS OR REPELLED 



FROM EACH OTHER. Hcncc, forcc miist be essentially either 

 attractive or repulsive in its character. By this action " energy " 

 is imparted to the matter put in motion : hence force may be 

 further cbaracterized as having the power of imparting energy. 

 But for the same reasons as those above stated, " the power 

 of imparting energy will not serve as a definition of force, 

 because energy may be imparted by other matter possessing 

 energy, without the intervention of any force. 



9. Cohesive attraction may be quoted as a force acting 

 between contiguous atoms or molecules of a body ; electric and 

 magnetic attraction and repulsion as forces acting between 

 certain particles and masses under certain conditions only ; 

 gravitation, or weight attraction, as a force acting indis- 

 criminately between all portions of matter : the mutual actions 

 of masses being only the aggregate of the actions of their 

 component particles. Heat, or more correctly speaking 

 thermic energy, is an universal source of repulsive force acting 

 between the particles of all kinds of matter. 



10. Energy was first (as the writer believes) defined by 

 Thomas Young to be The Power of Doing Work, and this 

 definition does not appear to require any amendment. 



11. Energy, as it exists in moving matter, is called actual or 

 kinetic : and this kind of energy implies the existence of 

 motion and vice versa, but it is not (as it has frequently been 

 assumed to be) identical or synonymous with motion. 



12. When energy, from the circumstances of the case, 

 remains undeveloped in matter, inactive but capable of being 

 called into action, it is termed potential energy .^^ Thus the 

 energy of chemical affinity existing between the elements of 

 gunpowder is j^otential ; but when called into action by 

 elevation of temperature, the repulsive force existing between 

 the particles of the highly-condensed and heated gases into 

 which the gunpowder is resolved imparts actual or kinetic 

 energy to the shot. 



13. If a weight be raised, a certain amount of energy is ex- 

 pended in raising it, and so long as the body is supported, the 

 energy expended in raising it remains potential in it ; but when 

 allowed to fall freely in vacuo to the level from which it was- 

 raised, the body acquires, in an active or kinetic form, exactly the 

 amount of energy that was expended in raising it. Similarly 



