Is there not a great charm in seeing an obscure subject clearly illuminated by facts well understood and well interpreted? If in a bottle containing wine and air and raised to a temperature of 50° or 60° the wine never turns sour, it is because the germs of the mycoderma aceti, which the wine and the air hold in suspension, are deprived of all vitality by the heat. Placed, however, in contact with ordinary air, this once-heated wine can turn sour; because, though the germs of the mycoderma aceti contained at first in the wine are killed, this is not the case with those derived from the surrounding air. Pure alcoholised water never turns sour, even in contact with ordinary air, and with whatever germs this air may carry, or that may be found in the dust of the vessels which receive it. The reason is that these germs cannot become fertile because of the absence of their indispensable food. Wine in bottles well filled and laid flat do not acetify; this is because the mycoderm cannot multiply for lack of oxygen. Without doubt the air constantly penetrates through the pores of the cork, but always in such feeble quantities that the colouring matters of the wine, and other more or less oxydisable constituents, take possession of it without leaving the smallest quantity for the germs of the mycoderm which are generally suspended in the wine. When the bottle is upright the conditions are quite altered. The desiccation of the cork renders it much more permeable to the air, and the germs of the mycoderm on the surface of the liquid, if any exist there, are enveloped by air. Thus, to recapitulate in a few words the principles which have just been established; it is easy to see that the formation of vinegar is always preceded by the development, on the surface of the wine, of a little plant formed of strangulated particles, of an extreme tenuity, and the accumulation of which sometimes takes the form of a hardly visible veil, sometimes of a wrinkled film of very slight thickness, and greasy to the touch, because of the various fatty matters which the plant contains. This cryptogam has the singular property of condensing considerable quantities of oxygen and of provoking the _fixation_ of this gas upon the alcohol, which is thereby transformed into acetic acid. The little mycoderm is not less exacting than larger vegetables. It must have its appropriate aliments. Wine offers them in abundance: nitrogenous matters, the phosphates of magnesia and of potash. The mycoderm thrives, moreover, in warm climates. To cultivate it in temperate regions like ours it is well to warm artificially the places where it is cultivated. But if wine contains within itself all the elements necessary to the life of the little mycoderm, this life is further promoted by rendering the wine more acid through the addition of acetic acid.