50 EXPERIMENTAL PHYSIOLOGY 



or decrease in either tube? After how long a time do you 

 get a test for grape sugar, and in which tube? Continue 

 these tests until you get a strong test for grape sugar. 



(The reason for these results is that the malt diastase — 

 the enzyme — ^is slowly changing' the starch into sugar. An 

 enzyme is a substance which can bring about the trans- 

 formation of one chemical compound, such as starch, into 

 another, such as sugar, without itself being used up. The 

 value of enzyme action in our bodies Ues in the fact that 

 by it a colloid, hke starch, may be changed into a crystalloid, 

 hke sugar, which can then be absorbed through a membrane 

 by dialysis, e, g., from the stomach through the walls of the 

 blood vessels into the blood.) 



XXXI. — ^A Ferment Organism — ^Yeast. 



Apparatus. — Yeast cake, molasses, eight-ounce bottle, absorbent 

 cotton, limewater, chemical thermometer. 



Directions. — Dissolve a piece of yeast cake, the size of 

 a pea, in two tablespoonfuls of water. Pour this into the 

 eight-ounce bottle. Add to this a tablespoonful of mo- 

 lasses and fill the bottle half full of water. Stopper with 

 a plug of absorbent cotton and leave in a warm place for 

 twenty-four hours. Record the temperature of the room 

 in which the bottle is put, and the temperature of the mix- 

 ture. 



At the end of the twenty-four hours remove the stopper 

 and examine the contents. What is the temperature? 

 Does it smell sweet? Test the gas in the top of the bottle 

 with a drop of lime water. What gas gives this reaction? 

 Does the odor give you any evidence of the presence of 

 alcohol? Examine under the low power of the compound 



