Page:Popular Science Monthly Volume 1.djvu/589

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573
YEAST.

But man is, here, greater as a mere dynamic than any other force acting on the globe. Already Niagara has felt his power. Fifty-two thousand cubic feet of water which belong to her, every summer minute be diverts to his own uses. Another century will see him on every acre along the borders of the upper lakes. Every forest he fells, every acre he ploughs, will affect, though inappreciably, the flow of water over the Falls. Time may come when his hand, laid on the earth in gigantic enterprise, will cause the Falls to shrink into insignificance. He will make these lakes furnish him highways to the ocean, east and south. A canal from Lake Michigan to the Illinois, great enough to float ships laden for the marts of Europe, and another from Lake Erie to Lake Ontario, are achievements in the near future.

 
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YEAST.
By THOMAS H. HUXLEY, LL. D., F. R. S.

IT has been known, from time immemorial, that the sweet liquids which may be obtained by expressing the juices of the fruits and stems of various plants, or by steeping malted barley in hot water, or by mixing honey with water, are liable to undergo a series of very singular changes, if freely exposed to the air and left to themselves, in warm weather. However clear and pellucid the liquid may have been, when first prepared, however carefully it may have been freed from even the finest visible impurities, by straining and filtration, it will not remain clear. After a time it will become cloudy and turbid; little bubbles will be seen rising to the surface, and their abundance will increase until the liquid hisses as if it were simmering on the fire. By degrees, some of the solid particles which produce the turbidity of the liquid collect at its surface into a scum, which is blown up by the emerging air-bubbles into a thick, foamy froth. Another moiety sinks to the bottom, and accumulates as a muddy sediment, or "lees."

When this action has continued for a certain time, with more or less violence, it gradually moderates. The evolution of bubbles slackens, and finally comes to an end; scum and lees alike settle at the bottom, and the fluid is once more clear and transparent. But it has acquired properties of which no trace existed in the original liquid. Instead of being a mere sweet fluid, mainly composed of sugar and water, the sugar has more or less completely disappeared, and it has acquired that peculiar smell and taste which we call "spirituous." Instead of being devoid of any obvious effect upon the animal economy, it has become possessed of a very wonderful in-