Page:Popular Science Monthly Volume 6.djvu/595

Rh I will tell you how you can prove that this is not true. These little torulæ float about in the air, or sleep in any dry place, never showing that they are alive till they are planted in some nest or nidus. When the cook dries her yeast-cakes, she puts all the little torulæ to sleep, and there they go into winter quarters, or hibernate in their cells, like the bears in their caves.

There is still another appearance of yeast. Let your cup of yeast stand long enough, and do not add any more sugar or water to it, you will find that the bubbling or fermentation stops, the torulæ settle to the bottom, and the fluid comes to the top. This fluid has a strong or biting instead of a sweet taste, like the fluid into which you first placed the yeast. The fermentation has changed its nature—the fairy torulæ with their magic wands have turned the sugar into carbonic acid, alcohol, glycerine, and succinic acid. These are called the products of fermentation. The carbonic acid, you know, passed off through the bubbles, the other products are still in the fluid. A little of this fluid will make you merry; if you take much of it, you will become intoxicated. This is due to the alcohol, and the value of yeast depends upon its power to make alcohol. You may know that the fluid is alcohol if, when you touch it with a lighted match, it burns with a blue flame.

Now, I have told you the torula grows—it has life; but how does it grow—as a mineral, a vegetable, or an animal? The mineral grows larger and larger by additions made to its outside. This is called growth by accretion. But the torula or yeast-cell grows by taking in new substance in among the particles of its old substance, and this kind of growth is called by a long name—intussusception. This is one of the reasons why it is not a mineral. Is it an animal? The line that divides the animal from the vegetable kingdom is not very well marked, but there are two reasons why the torula is not an animal. In the first place, its jelly or protoplasm is shut up in a close sac, but the protoplasm-jelly of animal cells forms a wall of itself. In the second place, the torula can make its own food or protein out of the raw material it finds in the liquid, while the animal cells seem to have no such power; they must have their protein already made, and