Everyone knows what is meant by _acid wine_, _sharp wine_, _sour wine_. The former experiments of Pasteur had clearly shown that no wine can become acid, sharp--can, in a word, become vinegar--without the presence of a little microscopic fungus known by the name of _mycoderma aceti_. This little plant is the necessary agent in the condensation of the oxygen of the air, and its fixation on the alcohol of the wine. Chaptal, who published a volume on the art of wine-making, knew of the existence of these mycoderm flowers; but to his eyes they were only 'elementary forms of vegetation,' which had no influence whatever upon the quality of the liquid. Besides the _mycoderma aceti_, which is the agent of acetification, there is another mycoderm called _mycoderma vini_. This one deposits nothing which is hurtful to the wine, and its flowers are developed by preference in new wines, still immature, and preserving the astringency of the first period of their fabrication. The requirements of the two sorts of flowers are such that even when the flower of vinegar is sown on the surface of a new wine, no development takes place. Conversely, the _mycoderma vini_ sown on wines that have grown old in casks or in bottles will refuse to multiply. The _mycoderma vini_ produces no alteration in the wine; it does not turn the wine acid. In proportion as the wine grows old the flower tends to disappear, the wine 'despoils' itself, to use a technical expression; physiologically speaking, the wine loses its aptitude to nourish the _mycoderma vini_, which, finding itself progressively deprived of appropriate nourishment, fades and withers. But it is then that the _mycoderma aceti_ appears, and multiplies with a facility so much the greater that it draws its first nourishment from the cells of the _mycoderma vini_. The _mycoderma aceti_ has played so large a part in the early pages of this book that it is not necessary to go back upon it here. There is another disease very common among wines when the great heat of summer begins to make itself felt in the vintage tubs. The wine is said to _turn_, to _rise_, to _spurt_. The wine becomes slightly turbid and at the same time flat and piquant. When it is poured into a glass, very small bubbles of gas form like a crown upon the surface. On placing the glass between the eye and the light and slightly shaking it, one can distinguish silky waves shifting about and moving in different directions in the liquid. When the _turned_ wine is in a cask, it is not unusual to see the bottom of the cask bulge a little and sometimes a leakage takes place at the joints of the staves. If a little opening is made, the wine spurts out, and that is the reason why the wine is said to spurt.