Essay on Motion and Force. 



267 



an inactive state and requiring certain conditions to arouse 

 it into action very naturally obtained. This latent force 

 was called vis mortua, an expression, it seems to me, the 

 most absurd that ever crept into the terminology of science ; 

 a term which conveys the idea of an incomplete cause. 

 " A cause of a phenomena," says Mill," is the antecedent, 

 or the concurrence of antecedents, upon which it is invari- 

 ably and unconditionally consequent :" yet in the term vis 

 mortua, we have the notion of force, (denned by Mayer to 

 be cause, and to which he says we should fully apply the 

 principle, " causa equat effectum)" which is not a force, and 

 which can not become a force or cause, until some other 

 condition occurs. The term vis viva as implying its oppo- 

 site vis mortua, is equally objectionable, and yet Mayer and 

 others make use of this term, and in such a way as to imply 

 the idea of a force, once inert, restored to activity. He says, 

 " the vis viva of the universe is a constant quantity." In 

 another place he says, " the attribute of every force is the 

 union of indestructibility with convertibility;" 'in another 

 place, " motion cannot be annihilated." !Now what be- 

 comes of motion, when vis viva is expended, and vis mortua, 

 or as he styles it, pushing force, supervenes ? For he also as- 

 serts that mere pushing force never produces motion. The 

 conversion of falling force into heat will not account for 

 the cessation of the aggerose motion of abody, when its mo- 

 tion is arrested by an elastic spring, for in this case heat mo- 

 tion does not appear. All these difficulties disappear if we 

 adopt the view that the supposed locked up force is molec- 

 ular motion. A ball laid gently upon the earth induces that 

 mode of molecular motion which is called tension, a term 

 which I consider as conveying the same notion as com- 

 pression or extension, according to the way in which the 

 motion, is communiated. If a ball falls rapidly to the earth, 

 upon the sudden cessation of its mass motion, both heat mo- 

 tion and tension take place, and these two motions, provided 

 no other motion takes place, will equal the entire motion of 

 the mass, previous to the change which takes place upon its 



