MECHANISM OF CHEMICAL CHANGES 159 



ing themselves a part of the final product, or without being 

 used up in the process of being used. For example, a 

 platinum sponge thrust into a jet of hydrogen will become 

 hot, finally glow and ignite the hydrogen. It has the 

 capacity of adsorbing upon its surfaces the hydrogen and 

 oxygen, and these are brought into such intimate contact 

 that they unite to form water, giving off heat. The platinum 

 apparently does not take part in the chemical change ; cer- 

 tainly it does not become a part of the final product, water. 

 Substances which can act in this fashion, that is, can bring 

 about chemical changes without themselves becoming part 

 of the final product and that are not used up by being used, 

 are produced by many cells, probably by all cells. Such 

 substances are termed enzymes. Enzymes, therefore, may 

 be somewhat inadequately defined as organic catalysts. 



Enzymes are undoubtedly extremely complex organic 

 compounds. They are relatively unstable, that is, they are 

 easily destroyed by unfavorable conditions. High tem- 

 peratures, excessive acidity or alkalinity, the presence of 

 certain chemicals, and light, all tend to destroy them. There 

 is, therefore, in any enzyme solution an' inevitable deteriora- 

 tion. 



It will be noted that the main function of enzymes is 

 apparently to bring about chemical changes. Methods of 

 isolating and identifying the enzymes themselves have not 

 proved successful in most cases at least. The exact chemical 

 composition of none of the enzymes is as yet known. In 

 fact there seems to be no chemical test which will enable 

 one to recognize the presence of enzymes other than that of 

 allowing the enzyme to act under suitable conditions, and 

 noting the changes brought about, the amount of enzyme 

 present being deduced from the amount of change pro- 

 duced. 



The action brought about by an enzyme is probably in 

 general reversible-, that is, an enzyme brings about a change 



