320 



PRINCIPLES OF GENERAL PHYSIOLOGY 



EQUILIBRIUM AND REVERSIBILITY 



Although the great majority of the reactions catalysed by enzymes are known 

 to be reversible, it is a matter of, at any rate, great theoretical interest to consider 

 how far it is justifiable to regard all reactions as reversible. According to 

 J. J. Thomson (1888, p. 281), if we take the view that the properties of matter 

 in motion are sufficient to account for all physical phenomena, irreversible 

 processes, such as those apparently made so lay frictional resistance, must be 

 capable of explanation as the combined effect of changes, all of which are them- 

 selves reversible. "It follows that, if we 'could only control the phenomenon 

 in all its details, it would be reversible ; so that, as was pointed out by Maxwell, 

 the irreversibility of any system is due to the limitation of our powers of 

 manipulation. The reason why we cannot reverse every process is because we 

 only possess the power of dealing with the molecules en masse and not individually, 



Fie. 86. EXTERIOR OF VA\'T HOFF'S OLD LABORATORY AT AMSTEKI>\M. 



(Reproduced by the kindness of Prof. Ernst Cohen 

 of the van't FToflf Laboratory, Utrecht. ) 



while the reversal of some processes would require the reversal of the motion of 

 each individual molecule." 



Nernst, also (1911, p. 442), states that reactions cannot be divided into 

 reversible and non-reversible. " There can be no doubt that by suitable adjust- 

 ment of the conditions of the experiment, it would be possible to make a reaction 

 take place, now in one direction, now in the opposite, that is, in principle every 

 reaction is reversible." One of the most obvious of these conditions referred to 

 is that of temperature, as in the well-known case of the dissociation of ammonium 

 chloride. 



It appears justifiable, then, to regard all reactions from this point of view 

 although, under ordinary conditions, the position of equilibrium may be so near 

 the state of complete change in one direction that the reaction seems to have gone 

 entirely in one direction. 



When a reaction is known to be reversible, it is customary to use the sign suggested by 

 van't Hoff(1884, p. 5), namely, two arrows pointing in opposite directions : 



acid + alcohol ~\ ester + water, 

 or more conveniently, by the modification of this suggested by H. Marshall (1902) : v ** 



We owe to van't Hoff (1884) the systematic investigation of the laws of the 

 velocity of reaction and of equilibrium. It may be of interest to the reader to 





