LAW OF CAUSATION. 253 



Now there is a real basis of fact for this supposition : we have positive evi- 

 dence of the existence of molecular motion in these manifestations of force. 

 In tlie 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 evidence is equally conclusive, since heat expands bod- 

 ies (that is, causes their particles to move from 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 collision 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 w-as supposed, 

 heat that causes the motion of particles, but the motion of particles that 

 causes heat ; the original cause of both being the previous motion (whether 

 molar or molecular — collision of bodies or combustion of fuel) which form- 

 ed the heating agency. This inference already contains hypothesis ; but at 

 least the supposed cause, the intestine motion of molecules, is a vera causa. 

 But in order to reduce the Conservation of Force to Conservation of Mo- 

 tion, it was necessary to attribute to motion the heat propagated, through 

 apparently empty space, from the sun. This required 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 constitutes 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 interstices of all bodies. The vibratory motion supposed to be tak- 

 ing place in the heated mass of the sun, is considered as imparted from 

 that mass to the particles of the surrounding ether, and through them to 

 the particles of the same ether in the interstices of terrestrial bodies ; and 

 this, too, with a sufficient mechanical force to throw the particles of those 

 bodies into a state of similar vibration, producing the expansion of their 

 mass, and the sensation of heat in sentient ci'eatures. All this is hypothe- 

 sis, though, of its legitimacy as hypothesis, I do not mean to express any 

 doubt. It would seem to follow as a consequence from this theory, that 

 Force may and should be defined, matter in motion. This definition, how- 

 aver, will not stand, for, as has already been seen, the matter needs not be 

 in actual motion. It is not necessary to suppose that the motion after- 

 ward manifested, is actually taking place among the molecules of the coal 

 luring its sojourn in the earth ;* certainly not in the stone which is at rest 

 Dn the eminence to which it has been raised. The true definition of Force 

 iiust be, not motion, but Potentiality of Motion ; and what the doctrine, 

 f established, amounts to, is, not that there is at all times the same quan- 

 tity of actual motion in the universe ; but that the possibilities of motion 

 vre limited to a definite quantity, which can not be added to, but which 

 ;an not be exhausted ; and that all actual motion which takes place in Na- 

 ure is a draft upon this limited stock. It needs not all of it have ever ex- 

 sted as actual motion. There is a vast amount of potential motion in the 

 miverse in the form of gravitation, which it would be a great abuse of 



* I believe, however, the accredited authorities do suppose that molecular motion, equiva- 

 3nt in amount to that which will be manifested in the combustion of the coal, is actually tak- 

 ng place during the whole of the long interval, if not in the coal, yet in the oxygen which 

 v'ill then combine with it. But how purely hypothetical this supposition is, need hardly be 

 emarked ; I venture to say, unnecessarily and extravagantly hypotheticaL 



