The author has conducted a number of experiments to demonstrate this action as follows: Material taken from the bottom of a pond in the fall after vegetation had died out was packed into a cylinder five feet long and six inches in diameter, water was added to within about 2 inches of the top. After leaving them open for a few days to permit all the dissolved oxygen to be used up by the aërobes, the cylinders were tightly capped and allowed to stand undisturbed. Pressure gauges reading to 500 lbs. were attached (Fig. 60). At the end of six months the gauge showed a pressure beyond the limits of the readings on it. Most of the gas was collected and measured 146 liters. An analysis of portions collected when about one-half had been allowed to escape showed the following composition, according to Prof. D. J. Demorest of the Department of Metallurgy: CO₂ 18.6 per cent. CH₄ 76.1 „ H 1.0 „ N 4.3 „ In the author’s opinion natural gas and petroleum have been formed in this way[12] (Figs. 61 and 62). [Illustration: FIG. 62.--A “flowing” oil well.] One of the very few practical uses of the gaseous fermentation of carbohydrates is in making “salt rising” bread. The “rising” of the material is due not to yeasts but to the formation of gas by certain bacteria which are present on the corn meal or flour used in the process (Fig. 63). [Illustration: FIG. 63.--A loaf of “salt rising” bread. The porous structure is due to the gas formed by bacilli and not by yeasts.] Another is in the formation of the “holes” or “eyes” so characteristic of Swiss and other types of cheese (Fig. 64). [Illustration: FIG. 64.--Ohio Swiss cheese. The “eyes” are due to gas formed by bacteria during the ripening of the cheese.] A great many organic acids are formed during the “acid fermentation” of carbohydrates by bacteria. Each kind of bacterium, as a rule, forms several different acids as well as other substances, though usually one is produced in much larger amounts, and the kind of fermentation is named from this acid. One of the commonest of these acids is lactic. The “lactic acid bacteria” form a very large and important group and are indispensable in many commercial processes. In the making of butter the cream is first “ripened,” as is the milk from which many kinds of cheese are made (Fig. 65). The chief feature of this “ripening” is the formation of lactic acid from the milk-sugar by the action of bacteria. A similar change occurs in the popular “Bulgarian fermented milk.” The reaction is usually represented by the equation: Milk-sugar. Lactic acid. C₁₂H₂₂O₁₁ + H₂O + (bacteria) = 4C₃H₆O₃