“The vintage is first of all subjected to a process of treading by men who trample on the grapes in large vats, after which the resulting mixture of juice and skins is left to ‘work,’ as we say. Before long this liquid mush begins to heat of its own accord, and presently there sets in a sort of boiling which liberates big bubbles of gas as if there were a fire underneath. This working process is called fermentation, and its seat is in the sugar of the grape-juice. Little by little the sugar decomposes, splits apart as we might say, into two substances very different from each other and also very different from the sugar whence they came. Of these two substances one is alcohol; the other is a gas already known to us—carbonic acid, the same gas that plants feed on and that animals give forth in breathing; the same, finally, as that produced by burning coal. The alcohol remains in the liquid, which thus gradually loses its original sweet taste and acquires instead a vinous flavor. The gas, on the contrary, works its way to the surface, agitating the mass with a sort of tumultuous movement like that of boiling water, and is dissipated in the atmosphere. “Let us bear in mind that carbonic acid gas is as invisible as the air itself, that it has no odor, no color, and finally that it kills quickly if inhaled in any considerable quantity. That explains the danger lurking in a wine-vat during fermentation, or even in a wine-cellar that lacks sufficient ventilation to carry off the perilous gas. No one should enter such places without holding before one a lighted taper at the end of a long stick. While the taper continues to burn in the usual manner, one can proceed without fear: there is no carbonic acid gas present. But if the flame becomes dim, gets smaller and smaller, and finally goes out altogether, one must beat a hasty retreat, for the extinction of the taper is a sure sign of the presence of carbonic acid gas, and further advance would mean exposing oneself to imminent death. “But to return to the subject of wine-making, we were saying that the sugar which imparts its sweet taste to the must (that is, the unfermented grape-juice) changes its nature and divides into two parts: alcohol, which remains in the liquid and turns it to wine, and carbonic acid gas, which is dissipated in the atmosphere. When this process is finished the wine is drawn off, leaving behind the residuum of skins and pips. The final product is thus composed of a large quantity of water from the grapes themselves, a small quantity of alcohol from the sugar which has undergone the chemical change just described, and, finally, a coloring substance furnished by the dark grape-skins.