12 GLANDS IN HEALTH AND DISEASE 



enzyme present may be there in relatively small 

 amount. Still more remarkable are such reactions 

 in that the enzymes do not seem to undergo any 

 permanent change : at the end of the reaction we 

 still find our enzymes, and for all the balance tells 

 us, in the same amounts as before the reaction. 



Substances that act like enzymes are called 

 "catalysts." A catalyst may be defined as a sub- 

 stance that accelerates a chemical reaction without 

 itself undergoing any permanent change. Hor- 

 mones and vitamines are probably catalysts. 



These catalysts are not confined to substances 

 that take part in the activities of the living organ- 

 ism. The best method for the manufacture of oil 

 of vitriol, or sulphuric acid, is by the employment 

 of platinum as a catalytic agent : provided all im- 

 purities are rigidly excluded, the same platinum 

 can be used over and over again. The manufacture 

 of synthetic ammonia by the Haber process in- 

 volves the use of a catalyst, probably nickel. I say 

 "probably" because the details of the process are 

 carefully guarded as trade secrets. At any rate, 

 the production of ammonia from the nitrogen of 

 the air, and the hydrogen that can be obtained 

 either from the electrolysis of water, or as a by- 

 product in the manufacture of lye, can be brought 

 about with the help of a catalytic agent. Since 

 this Haber process is on the road to solving the 

 "nitrogen-fixation" problem, and hence many of 



