METABOLISM 151 



At the temperature of 448 C., 79 per cent of 'the matter exists 

 as HI and the remainder as free H 2 and I 2 ; the HI is dissoci- 

 ated at the same rate at which the H 2 and I 2 unite, and a con- 

 dition of chemical equilibrium exists. At a different temper- 

 ature, the point of equilibrium is different, but otherwise the 

 result is the same. In theory, all chemical reactions are regarded 

 as reversible, but in many cases the reverse action is so slight as 

 to be incapable of detection under attainable experimental 

 conditions, and such reactions are often spoken of as irreversible. 



In addition to the temperature, the position of the point of 

 equilibrium in a reversible reaction is affected by the relative 

 mass of the ingredients. Thus if, in the example just given, 

 the iodin be removed from the field of chemical action (as, for 

 example, by condensing it to the solid form in a cold portion 

 of the apparatus) the dissociation of the hydrogen iodid will 

 proceed until it is practically complete. On the other hand, 

 if the hydrogen iodid be removed (as by allowing it to react 

 with calcium carbonate) the reaction may be pushed to 

 completion in the reverse direction. Similarly, an increase in 

 the concentration of one of the reacting substances tends to dis- 

 place the equilibrium in the opposite direction. 



It is a matter of much interest that at least some enzym 

 reactions have been shown to be reversible. One of the best 

 authenticated cases appears to be that of the action of lipase on 

 fats. It has been shown by Kastle and Loevenhart 1 that this 

 enzym acts on ethyl butyrate according to the equation 



C 2 H 5 C 4 H 7 O 2 + H 2 O ^ C 2 H 5 OH + C 4 H 8 O 2 



A similar reaction has also been shown to take place with 

 monobutyrin, the glycerol ester of butyric acid, which may be 

 regarded as a simple fat, while it is at least very probable that 

 the higher fats are acted on in the same way. Another example 

 of reversible enzym reaction is claimed to be that of the con- 

 version of maltose into dextrose by the action of the ferment 

 maltase, it appearing, according to the researches of Croft 

 Hill, 2 that the same ferment may also convert dextrose into 

 maltose. Similar, although less decisive, results have also been 

 reported regarding the action of the proteases. 



1 Amer. Qiem. Jour., 24 (1900), 491. 



2 Jour. Chem. Soc., Trans., 73 (1898), 634. 



