In a celebrated lecture given at Orleans at the request of the manufacturers of vinegar in that town, Pasteur, after having stated the two foregoing scientific questions, proceeded to examine the difference between wine and vinegar. What takes place in the fermentation of the juice of the grape which yields the wine? The sugar of this juice disappears, giving place to carbonic acid gas, which is exhaled during fermentation, and to alcohol, which remains in the fermented liquid. Formerly, chemists gave the name of 'spirit' to all volatile matters which could be collected from distillation. Now, when we distil wine and condense the vapour in a worm surrounded by cold water, we collect the spirit of wine at the extremity of the worm--this, when the water with which it is mixed during distillation is withdrawn from it, we designate by the name of alcohol. Vinegar contains no alcohol. When distilled it yields water and a spirit. But this spirit is acid, with a very pungent odour, and not inflammable like spirit of wine. Separated from the water which had accompanied it during the distillation, this spirit takes the name of acetic acid. This is the form in which it is used in smelling bottles--in those bottles of English salts the vapour of which is so penetrating. In the formation of vinegar in contact with air the alcohol disappears, and is replaced by acetic acid. The air has thus given up something to the wine. Atmospheric air every one knows to be a mixture of nitrogen and oxygen, the nitrogen in the proportion of four-fifths of the total volume, and the oxygen of one-fifth. Well, in the transformation of wine into vinegar the nitrogen remains inactive. It is the oxygen alone which enters into combination with the alcohol. You ask for the proof of this? Take a bottle of wine turned sour, a bottle which at the same time is stopped hermetically; if the oxygen of the air contained in the bottle has combined with the alcohol, then, instead of air, there will be nothing in the bottle but nitrogen gas. Turn the bottle upside down and open it in a basin of water. The water of the basin will rush into the bottle to fill the partial vacuum created by the disappearance of the oxygen. The volume of water which enters the bottle is precisely equal to a fifth part of the total original volume of the air which the bottle contained at the time when it was closed. Moreover, it is easy to show that the gas which remains in the bottle has the properties of nitrogen gas. A lighted match is extinguished in it as if plunged into water, and a bird dies immediately in it of asphyxia.