18 NATURE-STUDY REVIEW [12:1— Jan., 1916 



whether it is carbonic acid gas or nitrogen. Simply make a hole 

 in the center of the cork, insert one end of the tube, so that it 

 goes well beyond the bottom of the stopper, put the other end 

 into a tumbler of lime water, take off the rubber and cork the 

 bottle. There is no danger that the gas will escape when you 

 remove the rubber, because it is heavier than air. It takes about 

 half an hour for this experiment to work. 



This gas is produced also by baking powder, and is used in 

 making soda water. 



Sometimes the rubber top, after a day or two, is sucked into 

 the bottle instead of being inflated as before. This is because 

 the rubber when stretched allows some gas to escape, although 

 it does not permit air to take its place. After fermentation ceases 

 there is a rarefied atmosphere inside the bottle, and the oat- 

 side air forces in the rubber. 



Another important substance produced is alcohol, and there- 

 fore we call this particular kind of action alcoholic fermentation 

 to distinguish it from similar processes which produce different 

 results. Unfortunately we can not show the presence of alcohol 

 by a simple test, as we have just done with the gas. 



If the scum on the top of the liquid is examined under a micro- 

 scope it will be found to consist of yeast, most of which differs a 

 little from the plants described before. Some, it is true, are just 

 the same; but others have protuberances or buds growing from 

 their sides. When these buds become almost as large as the parent 

 plant they break off and start life for themselves. Thus the origi- 

 nal number of plants is greatly increased. This is the way in 

 which yeast is prepared for the market. It is grown in some 

 sweet liquid, is skimmed off the surface, drained, partly dried, 

 and mixed with some substance like flour whose starch will give 

 the cake body, will act as a dryer and will make the yeast keep 

 better in hot weather. You will find that the covering of Fleisch- 

 mann's yeast states the particular kind of flour used in that cake. 

 Dry yeast contains less water than compressed yeast. It keeps 

 longer, but is slower in action. 



But, after all, it is not the growth of the plant which causes 

 this alcoholic fermentation, for it has been found that yeast which 

 has been ground up with sand and then subjected to great pres- 

 sure in a bag will give up a liquid which when strained and given 

 the proper conditions of "food" and temperature will carry on 



