A STUDY OF LIVING SUBSTANCE 35 



settle down on a moist surface that will supply them with 

 food, the spores burst their thick envelope, assume once 

 more their rod-shaped or spiral form, and go on feeding, 

 assimilating, and reproducing their kind. 



Many forms of bacteria feed upon dead animals and 

 plants, cause them to decay, and thus finally convert them 

 into carbon dioxid, water, and other simple compounds that 

 can be used by the higher plants. In this way these micro- 

 organisms are of incalculable benefit to mankind and to all 

 forms of life, for they keep the surface of the earth from 

 becoming a vast cemetery. To the action of bacteria also 

 are due many of the flavors of meats and cheese. 



Unfortunately, however, there are certain germs that find 

 favorable conditions for growth only in living animal tissue. 

 Thus the bacterium of consumption grows in the lungs, the 

 germ of diphtheria in the throat, and the bacteria that cause 

 typhoid fever in the intestines. These disease-producing 

 germs are called by Dr. Prudden "Man's invisible foes." 

 Yet wonderful progress is being made in the fight against 

 them. We have learned how to check the ravages of chol- 

 era, typhoid, and diphtheria, and even consumption is found 

 to be a preventable disease. Further discussion of bacteria 

 will be found in several of the subsequent chapters. 



5. A STUDY OF YEAST AND FERMENTATION 1 



Changes Caused by Yeast. A small piece of a cake of com- 

 pressed yeast, mixed in a spoonful of water, forms a milky 

 fluid that is much like so-called bakers' or brewers' yeast. 

 When this is added to a cupful of water, in which a spoon- 

 ful of molasses has been dissolved, one has a convenient 

 mixture with which to carry on experiments in fermentation. 2 



1 See "Laboratory Exercises," No. 48. 



2 This mixture should be placed in a loosely stoppered bottle, for if 

 the bottle is tightly closed, an explosion may be caused by the pressure 

 of the gas that is formed. 



