YEAST 573 



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 in- 

 sionificance. 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. 



YEAST. 



By THOMAS II. HUXLEY, LL.D., F.E.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 moi- 

 ety 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 watei*, 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- 



