166 VELOCITY OF REACTION, AND 



and the energy so set free is the driving-force towards equilibrium, 

 or a tendency towards chemical equilibrium. But whether there 

 will be any movement towards equilibrium or the reverse, and the 

 velocity of that movement, depend entirely upon how far there 

 is opposition to such movement, by the resistance of other forms 

 of energy present in or brought to bear upon the system by any 

 agency such as a catalyst, or upon how far such resisting forms of 

 energy are diminished by the action of any such agent which may 

 be present in the system. 1 



Chemical energy is not peculiar in this respect, and does not 

 stand isolated from other forms of energy. Water standing in two 

 reservoirs, unconnected by a channel through which the water 

 can flow, will remain at a constant difference in hydrostatic pressure 

 for ever, and if there be a channel of communication between them, 

 the rate of flow that is, the velocity with which the potential 

 difference is equated out will vary not only with the difference 

 in potential but with the resistance to the flow of water in the con- 

 necting channel. Further, if there is a turbine, or properly con- 

 structed reversible pump, on the channel of communication between 

 the two reservoirs, then the water as it flows from the higher to the 

 lower level can be made to give out energy which by suitable 

 transforming machines can be changed into any form of energy; 

 or conversely, if external energy is supplied to the pump, at a 

 sufficiently high potential, it can be made to work the pump in the 

 opposite direction, and the absorbed energy can so be utilised to 

 force the water in the opposite direction from the lower reservoir to 

 the higher reservoir, so that the difference in hydrostatic potential 

 increases instead of diminishing, as it would do if the system were 

 not operated upon by other forms of energy from without. 



Similarly in the case of electrical energy, if there is no path of 

 conduction between two charged conductors at different potentials, 

 there can be no equalisation of potential between the two con- 

 ductors; if a path is provided the velocity of the energy reaction 



1 Lest the reader should think that, because these equations do not lead 

 directly to expressions for the velocity of reaction, they are therefore use- 

 less, it may be pointed out that they do give the conditions for equilibrium 

 quite truly when no energy is imparted to the system from without. For 

 although variations in resistance will alter the velocity with which equilibrium 

 is reached, at the equilibrium point itself the velocity becomes zero, and the 

 resistance has accordingly no effect upon the equilibrium point. 



