Nominated at the age of thirty-two as Dean of the Faculté des Sciences, at Lille, Pasteur determined to devote a portion of his lectures to fermentation. At that time ferments were believed to be, to quote Liebig, "Nitrogenous substances--albumin, fibrin, casein; or the liquids which embrace them--milk, blood, urine--in a state of alteration which they undergo in contact with air." Pasteur examined the lactic ferment and found little rods, 1/25000 inch in length, which nipped themselves in the centre, divided into two, grew to full length and divided again, and these living things he declared to be the active principles of the ferment. He made a mixture of yeast, chalk, sugar, and water, added some of the rods, and got fermentation. He then made a mixture of sugar, water, phosphate of potash, and magnesia, and introducing fresh cells, fermentation followed. Liebig's theory of the nitrogenous character of the ferment disappeared when fermentation was caused in a mixture having no nitrogenous elements. Pasteur had discovered that fermentation was a phenomenon of nutrition; it followed the increase and growth of the little rods. The next step was the discovery of the ferment of butyric acid, a species of vibrio consisting of little rods united in chains of two or three and possessed of movement. He found these vibrios lived without air. Further experiments showed there were ferments to which air was necessary, called by Pasteur the _ærobics_, and others to whom oxygen was fatal, the _anærobics_. He proved, also, by an exhaustive series of experiments, that what is called putrefaction of animal matter is the result of the combined work of the _ærobics_ and the _anærobics_, which reduce that part not taken up by oxygen to dead organic matter, ready in its turn to form food for living things. His attention having been turned to the needs of the vinegar makers of Orleans, Pasteur began the examination of the ferment which produces vinegar from wine. He found this in the mycoderm aceto, a mould-like plant which has the power of developing acetic acid from alcohol. As the result of his investigation, the manufacturers of vinegar in France were able to do away with the cumbrous process they had long followed, and to make vinegar, not only more cheaply, but of very much better quality. But during these experiments Pasteur found the temperature of 65° C. was sufficient to kill the mycoderm. When, then, the wine makers of France appealed to him to investigate the "diseases" of wine, he was ready for the work. Before this, however, he had examined the claims of Pouchet and others to their alleged discovery of spontaneous generation; in other words, the production of life. Ranging himself against them, Pasteur showed their experiments not to have been conclusive, simply because they had not succeeded in excluding the dust which contained germs of life in the shape of spores of microscopic plants.