[[Categoria:Pagine che usano RigaIntestazione|Scientia - Vol. VIII.djvu{{padleft:316|3|0]]

If we take a general view of the chemical reactions occurring in living organisms, the fact that strikes us most is the large number of processes going on at moderate temperatures and in media of nearly neutral reaction, which require in the laboratory high temperatures or strong acids to cause them to take place at similar rates. Such reactions are, oxidations of sugars and fats to carbon dioxide and water, the hydrolysis of proteins to amino-acids, of starch to sugar, and many others.

There is, however, a certain class of laboratory reactions, known as « catalytic », in which such changes are produced by what may be called comparatively gentle means. For example, lactic acid is oxidized to water and carbon dioxide by hydrogen peroxide at a very slow rate; but if a trace of an iron salt be added, the reaction proceeds very rapidly. This is known as « Fenton’s reaction », and has been made much use of. In this case the iron salt is called the « catalyst » and we may notice at once some of the characteristic properties of these bodies. In the first place, the reaction proceeds, although slowly, without the aid of a catalyst and is merely greatly accelerated by this agent. In the second place, the catalyst does not form a constituent of the final result of the reaction, in the sense that it is unchanged and capable of being used for further work. This latter fact shows that it neither adds energy to the reacting system nor withdraws energy from the system.