FERMENTATION AND GERM DISEASES 67 



"swell"; Figs 29 and 30. Alcohol also is produced, but passes 

 off when the bread is baked. The use of baking soda in pre- 

 paring food materials produces a lightness due to bubbles 

 also; but only the water in the mass is affected by its presence. 

 When a brewer makes beer, he makes a solution from certain 

 grains, containing starch which is changed into sugar, and 

 plants yeast in it. The yeast grows and destroys the sugar, 

 producing alcohol and carbonic acid gas. In the making of 

 cider or wine, when the sweet juice from apples or grapes is 

 squeezed out and placed in barrels, the wild yeasts from the 

 air grow in it, producing the same changes. 



BACTERIA 



Bacteria are more abundant than yeasts and are even 

 smaller (Fig. 31); they are so small that sometimes 50,00tr 

 of them, side by side, 

 would reach only an inch. 

 But small as they are, 

 they play a very impor- 

 tant part in our lives and 

 in the world. It may seem 

 strange that organisms as FlG - 31. SHOWING THE COMPARATIVE 

 minute as these can have SIZES OF THE PO ' NT OF THE FINEST 



. . . CAMBRIC NEEDLE (a). A PARTICLE OP 



an appreciable effect. A DUST (&), BACTERIA (c), AND YEASTS (d) 



single one, to be sure, 



could do very little; but bacteria have such wonderful powers 

 of reproduction that they can accomplish much. So fast 

 do they multiply that, under favorable conditions, in twenty- 

 four hours a single one may have seventeen million offspring, 

 and in another twenty-four hours each of the first seventeen 

 millions may have seventeen million more. Most people 

 think of bacteria, germs or microbes as our deadly foes. While 

 this is partly true, it is likewise true that they are our friends. 

 Their distribution in the world shows clearly enough that they 



