The process by which grape and other saccharine vegetable juices are converted after much frothing, turbidity, and ultimate clarification, into an intoxicating drink is as old as the history of mankind. But the true nature of the change which goes on was established only after a long controversy, in which the representatives of a purely mechanical or physical theory originated by Liebig were finally defeated by Pasteur, who established the dependence of ordinary alcoholic fermentation on the action of the living yeast cell.