THE FUNGUS, OR COLORLESS PLANT. 23 



lieat has killed all the torul^ that were inside, and the 

 " cotton-wool " acts as a strainer, and will not allow any 

 of these germs or cells which might be floating in the 

 air to pass through it. I have seen bottles more than a 

 year after they had been corked in this way, and yet 

 there was no sign of bubbling or fermentation in the 

 fluid. If the cells could make themselves, surely, in 

 such a bottle they have time and opportunity. Cotton 

 is the best air-strainer that has ever been discovered ; if 

 you are ever in a place where the air is tilled with fine 

 mineral dust or poisonous matter, you can prevent it 

 from entering your lungs by breathing through a piece 

 of cotton. Such a filter is called a respirator. Com- 

 mon clay jars, like your mammas' flower-pots, are also 

 good filters for the yeast. These jars are full of tiny 

 holes, or pores, too small to be seen. If you put some 

 yeast in one of these porous jars, and set this one in a 

 larger jar containing sweetened water, the yeast-fluid 

 will pass out and the sweet-fluid will pass in ; the two 

 will thus mix together (Fig. 16), but there will be no 

 bubbling or fermentation in the outer jar, because the 

 torul^e cannot pass through the pores or little holes of 

 the jar, the cells are so much larger than the holes. 



These little yeast-cells float about in the air, or lie 

 asleep in any place where it is dry and comfortable, and 

 never show that they are alive or awake till they are 



