THE CHEMISTRY OF THE DIGESTIVE /VICES 339 



Many of the ordinary substances of the laboratory will accelerate 

 reaction which goes on slowly in their absence. These are called 

 catalysers. Some writers also speak of catalysers which retard 

 a reaction progressing quickly in their absence. The process by 

 which the reaction is accelerated (or retarded) is termed catalysis. 

 A typical catalyser can exert its action when it is present in ex- 

 ceedingly small amount in comparison with the substance acted 

 upon. However it may enter into the reaction, it does not take 

 part in the formation of the final products nor contribute to the 

 energy changes, and for this reason is often apparently unaltered 

 at the end of the process. The catalysers have therefore been 

 compared to the lubricants used for machinery as contrasted with 

 the coal or other source of energy. If it be remembered that the 

 expression is a purely metaphorical one, we may say that the 

 catalyst oils the reaction so that it slips on smoothly and swiftly 

 to an end-point which would, however, have been reached just the 

 same in time. A classical instance of catalysis is the inversion 

 of cane-sugar by weak acids, i.e., the change of the cane-sugar into 

 a mixture of equal quantities of dextrose and levulose a reaction 

 which may be represented by the equation 



= C 6 H 12 O 6 + C 6 H 12 O 6 . 



Cane-sugar. Water. Dextrose. Levulose. 



This is a reaction which occurs also when the sugar is simply dis- 

 solved in water, but with extreme slowness at the ordinary tempera- 

 ture, although more rapidly at 100 C. The effect of the acid is 

 to catalyse the reaction, to markedly accelerate it. The hydrogen 

 ions of the free acid are responsible for the catalysis, and they are 

 not used up in the process, for the reaction at the end is unaltered. 

 The same action upon cane-sugar is exerted by an enzyme, invertase, 

 found in intestinal juice, although the laws governing the reaction 

 are somewhat different. Reversibility of the reaction can be even 

 more clearly demonstrated for catalysers than for enzymes. For 

 example, the condensation of acetone to diacetone-alcohol, which 

 is accelerated by hydroxyl ions (as by the addition of sodium 

 hydroxide, ammonia, etc.), only proceeds to a certain point, at which 

 equilibrium is established between the proportions of acetone and 

 the condensation product. Henceforth as much of the latter is 

 decomposed as is condensed. Thus : 



2CH 3 .CO.CH3^=H>CH 8 .cb.CH 2 .C(CH 3 ) 2 OH. 



Acetone. Acetone alcohol. 



On the other hand, the final equilibrium point need not be the same 

 for a catalyser and an enzyme. For example, amyl butyrate is 

 formed and decomposed according to the equation 



C 6 H U OH +C 3 H 7 COOH=II?' CgHyCOO.CgHn +H 2 O. 



Amyl alcohol. Butyric acid. Amyl butyrate. Water. 



