12 



BOYS AND GIRLS IN BIOLOGY. 



apple-tree. I dare say you all know the story. It is 

 this same attraction which keeps the air down to the 

 earth, and does not let it fly away. How funny it 

 would be to see boys' caps, and balls, and even the 

 boys themselves, go flying up into the sky ! What is 

 attraction ? I do not know ; you sometimes get your 

 knives magnetized, and then, when you put the knife 

 near a nail, the nail comes right toward it. The force 

 that draws the knife is one kind of attraction. The 

 centre of the earth is something like the magnetized 

 knife ; it makes every thing on the surface come tow- 

 ard it, and keeps us all from flying away nobody knows 

 where. So much for the gas that makes the yeast- 

 bubbles. The next thing you will notice is the brown- 

 ish color of the yeast. As it stands, it grows thicker and 

 more muddy-looking, and, in the course of an hour or 

 so, it begins to rise, as the cook says ; the chemists call 

 this rising fermentation, and the biologists call it 

 growing. You will find that your spoonful has grown 

 into a cupful running over. It is just as though a 

 farmer should sow a handful of wheat, and, for every 

 grain of wheat he sows, he finds a hundred grains in 

 the harvest. So you sow the yeast in the sugar and 

 water, which is its soil, or nidus, as the scientific men 

 call it, and you have all this harvest of new yeast. Per- 

 haps you are ready now to understand why the profes- 



