An acid, called acetic acid, and water.
If wine or beer be imperfectly corked, why does it rapidly turn sour?
Because air gets into the liquor, and the oxygen of the air combining with the alcohol of the liquor produces acetic acid, or vinegar.
What is alcohol?
Alcohol is the spirit existing in wine, beer, cider, etc., obtained in the process of fermentation.
What is a ferment?
A ferment is a substance containing nitrogen in a state of decomposition, which is able to excite fermentation in solutions of sugar; old cheese, putrefying flesh, blood, etc., all of them are ferments.
What is yeast?
We apply the term yeast to a particular species of ferment; the foam of beer (or of some similar liquor), produced by fermentation.
Can you explain why it is that a body in a state of fermentation or putrefaction should cause unlimited quantities of similar matter to pass into the same state?