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



a bubble in it, is the yeast plant. The starch grains are added 

 principally for mechanical reasons, which will be discussed 

 later. 



We all know the final result of adding yeast to dough ; but we 

 cannot there watch the inside processes that go on, nor can we 

 readily test any of the substances formed. It is easy enough, 

 however, to make a more transparent mixture which will let us 

 look behind the scenes. At the same time we can try other 

 materials and various conditions, and can thus draw our own 

 conclusions as to circumstances favoring or retarding the growth 

 of the plant. 



For these experiments we should get if possible six wide-mouthed 

 little bottles of the same size, and some thin rubber tissue of the 

 kind unpleasantly associated with dentists. In making up the 

 contents of the bottles we may experiment with representatives 

 of the classes of foods : starch and molasses as different carbohy- 

 drates, raw white of egg as a nitrogenous food, butter as a fat, 

 salt as a mineral, and water. The following directions will show 

 one way of treating the subject. 



Make a weak solution in water of each of the first five foods, 

 allowing one teaspoonful to nine teaspoonmls of water. Put 

 each solution into its own bottle, filling the bottle only half full. 

 Into each put about one-sixteenth of a yeast cake and cover with 

 a square of rubber, stretched a little and tied securely over the 

 mouth. Mark the bottles with the names of their contents and 

 leave them in the kitchen over night. If you have no rubber, 

 cork the bottles. 



In the morning set aside unopened the bottle which shows the 

 most action and notice what is happening. The name of this 

 process is fermentation. Of the foods tested this particular kind, 

 then, is the best for awakening the plant's activities. Let us 

 therefore throw away the contents of the other bottles, wash 

 them and use four of them in an attempt to find out some of the 

 conditions affecting this sort of fermentation. 



We now need enough of the food solution for three bottles. 

 The first one will receive yeast and will be covered with rubber 

 and placed in the ice-box. The second will receive yeast and will 

 be covered first with rubber and then with thick, dark paper 

 arranged so as to keep out light, but not air. It and the third 

 bottle, which is exactly like the one left over from the day before, 



