It was on the Monday, November 11, at 7.30 p.m., that Pasteur entered the lecture room at Orleans. A great many vinegar manufacturers, some doctors, apothecaries, professors, students, even ladies, had come to hear him. An account in a contemporary local paper gives us a description of the youngest member of the Académie des Sciences as he appeared before the Orleans public. He is described as of a medium height, his face pale, his eyes very bright through his glasses, scrupulously neat in his dress, with a tiny Legion of Honour rosette in his button hole. He began his lecture with the following simple words: “The Mayor and the President of the Chamber of Commerce having heard that I had studied the fermentation which produces vinegar, have asked me to lay before the vinegar makers of this town the results of my work. I have hastened to comply with their request, fully sharing in the desire which instigated it, that of being useful to an industry which is one of the sources of the fortune of your city and of your department.” He tried to make them understand scientifically the well known fact of the transformation of wine into vinegar. He showed that all the work came from a little plant, a microscopic fungus, the _mycoderma aceti_. After exhibiting an enlarged picture of that mycoderma, Pasteur explained that the least trace of that little vinegar-making plant, sown on the surface of any alcoholic and slightly acid liquid, was sufficient to produce a prodigious extension of it; in summer or artificial heat, said Pasteur, a surface of liquid of the same area as the Orleans Lecture room could be covered in forty-eight hours. The mycodermic veil is sometimes smooth and hardly visible, sometimes wrinkled and a little greasy to the touch. The fatty matter which accompanies the development of the plant keeps it on the surface, air being necessary to the plant; it would otherwise perish and the acetification would be arrested. Thus floating, the mycoderma absorbs oxygen from the air and fixes it on the alcohol, which becomes transformed into acetic acid. Pasteur explained all the details in his clear powerful voice. Why, in an open bottle, does wine left to itself become vinegar? Because, thanks to the air, and to the mycoderma aceti (which need never be sown, being ever mixed with the invisible dusts in the air), the chemical transformation of wine into vinegar can take place. Why does not a full, closed bottle become acetified? Because the mycoderma cannot multiply in the absence of air. Wine and air heated in the same vessel will not become sour, the high temperature having killed the germs of mycoderma aceti both in the wine itself and in the dusts suspended in the air. But, if a vessel containing wine previously heated is exposed to the free contact of ordinary air, the wine may become sour, for, though the germs in the wine have been killed, other germs may fall into it from the air and develop.