252 INDUCTION. 



foot, or a weight of one pound 772 feet: and the same exact quantity of 

 heat can, by certain means, be recovered, through the expenditure of exact- 

 ly that amount of mechanical motion. 



The establishment of this comprehensive law has led to a change in the 

 language in which the scientific world had been accustomed to speak of 

 what are called the Forces of nature. Before this correlation between phe- 

 nomena most unlike one another had been ascertained, their unlikeness had 

 caused them to be referred to so many distinct forces. Now that they are 

 known to be convertible into one another without loss, they are spoken of 

 as all of them results of one and the same force, manifesting itself in dif- 

 ferent modes. This force (it is said) can only produce a limited and defi- 

 nite quantity of effect, but always does produce that definite quantity; and 

 produces it, according to circumstances, in one or another of the forms, or 

 divides it among several, but so as (according to a scale of numerical 

 equivalents established by experiment) always to make up the same sum ; 

 and no one of the manifestations can be produced, save by the disappear- 

 ance of the equivalent quantity of another, which in its turn, in appropriate 

 circumstances, will re-appear undiminished. This mutual interchangeabil- 

 ity of the forces of nature, according to fixed numerical equivalents, is the 

 part of the new doctrine which rests on irrefragable fact. 



To make the statement true, however, it is necessary to add, that an in- 

 definite and perhaps immense interval of time may elapse between the dis- 

 appearance of the force in one form and its re-appearance in another. A 

 stone thrown up into the air with a given force, and falling back immedi- 

 ately, Avill, by the time it reaches the earth, recover the exact amount of me- 

 chanical momentum which was expended in throwing it up, deduction be- 

 ing made of a small portion of motion which has been communicated to 

 the air. But if the stone has lodged on a height, it may not fall back for 

 years, or perhaps ages, and until it does, the force expended in raising it is 

 temporarily lost, being represented only by what, in the language of the 

 new theory, is called potential energy. The coal imbedded in the earth is 

 considered by the theory as a vast reservoir of force, which has remained 

 dormant for many geological periods, and will so remain until, by being 

 burned, it gives out the stored-up force in the form of heat. Yet it is 

 not supposed that this force is a material thing which can be confined by 

 bounds, as used to be thought of latent heat when that important phenom- 

 enon was first discovered. What is meant is that when the coal does at 

 last, by combustion, generate a quantity of heat (transformable like all oth- 

 er heat into mechanical momentum, and the other forms of force), this ex- 

 trication of heat is the re-appearance of a force derived from the sun's rays, 

 expended myriads of ages ago in the vegetation of the organic substances 

 which were the material of the coal. 



Let us now pass to the higher stage of the theory of Conservation of 

 Force ; the part which is no longer a generalization of proved fact, but a 

 combination of fact and hypothesis. Stated in few words, it is as follows : 

 That the Conservation of Force is really the Conservation of Motion ; that 

 in the various interchanges between the forms of force, it is always motion 

 that is transformed into motion. To establish this, it is necessary to as- 

 sume motions which are hypothetical. The supposition is, that there are 

 motions which manifest themselves to our senses only as heat, electricity, 

 etc., being molecular motions ; oscillations, invisible to us, among the mi- 

 nute particles of bodies ; and that these molecular motions are transmutable 

 into molar motions (motions of masses), and molar motions into molecular. 



