A process by which reaction occurs in the presence of
   certain agents which were formerly believed to exert an influence by
   mere contact. It is now believed that such reactions are attended with
   the formation of an intermediate compound or compounds, so that by
   alternate composition and decomposition the agent is apparenty left
   unchanged; as, the catalysis of making ether from alcohol by means of
   sulphuric acid; or catalysis in the action of soluble ferments (as
   diastase, or ptyalin) on starch.