LAW OF CAUSATION. 



229 



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 tem- 

 porarily lost, being represented only 

 by what, in the language of the new 

 theory, is called potential energy. The 

 coal imbedded in the earth is con- 

 sidered by the theory as a vast reser- 

 voir of force, which has remained 

 dormant for many geological periods, 

 and will so remain until, by being 

 burnt, 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 phenomenon was 

 first discovered. What is meant is 

 that when the coal does at last, by 

 combustion, generate a quantity of 

 heat, (transformable like all other heat 

 into mechanical momentimi and the 

 other forms of force,) this extrication 

 of heat is the reappearance 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 gene- 

 ralization of proved fact, but a com- 

 Innation of fact and hypothesis. 

 Stated in a few words, it is as follows : 

 That the Conservation of Force is 

 really the Conservation of Motion ; 

 that in the various interchanges be- 

 tween the forms of force, it is always 

 motion that is transformed into mo- 

 tion. To establish this, it is necessary 

 to assume motions which are hypo- 

 thetical. The supposition is, that there 

 are motions which manifest themselves 

 to our senses only as heat, electricity, 

 &c., being molecular motions ; oscilla- 

 tions, invisible to us, among the minute 

 particles of bodies ; and that these 

 molecular motions are transmutable 

 into molarmotions (motions of masses) 

 and molar motions into molecular. 

 Now there is a real basis of fact for 

 this supposition : we have positive 

 evidence of the existence of molecular 



motion in these manifestations of 

 force. In the case of chemical action, 

 for instance, the particles separate 

 and form new combinations, often 

 with a great visible disturbance of the 

 mass. In the case of heat, the evi- 

 dence is equally conclusive, since heat 

 expands bodies (that is, causes their 

 particles to move froni one another) ; 

 and if of sufficient amount, changes 

 their mode of aggregation from solid 

 to liquid, or from liquid to gaseous. 

 Again, the mechanical actions which 

 produce heat — friction, and the col- 

 lision of bodies — must from the nature 

 of the case produce a shock, that is, 

 an internal motion of particles, which 

 indeed, we find, is often so violent as 

 to break them permanently asunder. 

 Such facts are thought to warrant the 

 inference that it is not, as was sup- 

 posed, heat that causes the motion of 

 particles, but the motion of particles 

 tliat catiscs heat ; the original cause 

 of both being the previous motion 

 (whether molar or molecular — col- 

 lision of bodies or combustion of fuel) 

 which fOTmed the heating agency. 

 This inference already contains hypo- 

 thesis : but at least the supposed 

 cause, the intestine motion of mole- 

 cules, is a vein causa. But in order 

 to reduce the Conservation of Force 

 to Conservation of Motion, it was 

 necessary to attribute to motion the 

 heat propagated, through apparently 

 empty space, from the sim. This re- 

 quired the supposition (already made 

 for the explanation of the laws of 

 light) of a subtle ether pervading 

 space, which, though impalpable to us, 

 must have the property which consti- 

 tutes matter, that of resistance, since 

 waves are propagated through it by 

 an impulse from a given point. The 

 ether must be supposed (a supposition 

 not required by the theory of light) 

 to penetrate into the minute inter- 

 stices of all bodies. The vibratory 

 motion supposed to be taking place in 

 the heated mass of the sun, is con- 

 sidered as imparted from that mass 

 to the particles of the surrounding 

 ether, and through them to the par- 



