ACTION OF ENZYMES AND CELLS 167 



varies with the resistance of the path; also by giving various 

 forms to the conducting path, the electrical energy may be 

 transformed into various forms ; and lastly, if a sufficiently powerful 

 dynamo be placed upon the conducting path and worked in the 

 right direction, instead of the path being a means for equating 

 potential it is converted into a means for heaping up difference 

 in potential. 



So also for any type of mechanical engine or motor, however 

 driven: if the load is taken off, the engine races; as the load is in- 

 creased the velocity lessens, and with a sufficiently heavy load the 

 movement stops entirely. Always when the potential differences 

 of the opposing forms of energy become greater than those of the 

 form of energy driving the motor, the latter becomes ineffective and 

 the engine must stop, or, if built so as to be capable of reversal, 

 must run in the opposite direction. 



Exactly similar reasoning applies to every known type of energy, 

 and since the law of conservation of energy holds and a definite 

 amount of chemical energy is equivalent to a definite amount of any 

 other form, it is clear that the reasoning must hold for chemical 

 energy also. 



Hence we see that while the difference in chemical energy gives 

 the driving force tending to cause chemical reaction, and a passage 

 towards a definite point of equilibrium, there is present something 

 in the nature of a resistance or load upon the engine, which deter- 

 mines by its amount whether a reaction shall occur at all, if it occurs 

 the speed at which it shall occur, and according as the resistance is 

 modified by other factors, the path of the reaction is determined 

 and the very qualitative nature of the compounds formed by the 

 reaction. 



It is hence necessary for our purpose to inquire what is the 

 nature of the resistance to chemical reaction, what are the forms 

 of energy opposed to the reaction, and how is the action of these 

 opposing forms of energy altered under different circumstances, so 

 that the velocity of reaction becomes changed, the reaction stopped 

 or its actual direction reversed, or finally the path of the reaction 

 altered so that, under different conditions, different products may 

 be formed. 



The obvious forms of energy opposed to chemical reaction are : 

 (1) molecular cohesion or chemical affinity, which must be over- 

 come before the molecule breaks up or is rendered capable of react- 



