Among the many fermentations there is one which demands special attention on account of its consequences to mankind: one of its products is the liquid known as alcohol—a poison which is the main constituent of all commonly used intoxicating drinks. A poison is any substance which tends when taken into the body to injure or kill it.

By very roundabout and expensive methods chemists can make alcohol without the aid of a ferment; but in so far as alcoholic drinks are to be found in the bar-room or drugstore for sale, they have all (whether beer, cider, ale, porter, wine, whiskey, brandy, cordials, or "tonics") been made by the action of ferments known as yeast. The substance commonly known as bakers' or brewers' yeast is only one kind of the yeast or ferment which is the cause of alcoholic fermentation.*

State a use of fermentative organisms. What substance do all ordinarily used intoxicating drinks contain ? What living ferment produces alcohol ? What is a poison ?

* Another very useful name now coming into use is that of enzyme for a dead ferment such as pepsin, the name ferment being restricted to formed ferments.

The yeast plant cannot thrive in strong solutions of sugar, and therefore preserves and syrups may "be kept for a long time without fermentation. But in weak solutions of sugar the yeast plant thrives and turns the sugar into carbon dioxide and alcohol. †