XXXIII. BACTERIA AND SANITATION. 



Whenever moist foods, such as milk, eggs, meats, bread, 

 etc., are exposed to warm air for any length of time changes 

 take place in their composition which render them unfit for 

 food. These changes we designate as souring, fermentation, 

 decay, mold formation, etc., according to the character of 

 the change. Such changes are produced hot by the action 

 of the air, but by the presence in the air of certain vegetable 

 organisms commonly called germs. Analysis of these 

 germs shows that there are three forms of plants which may 

 cause changes in food, and that these three forms belong to 

 the classes yeasts, molds, or bacteria. While these three 

 forms of plants are very different in their appearance and 

 action, they show striking similarity in the manner in which 

 they bring about changes in foods. Of these three forms, 

 the bacteria are perhaps the most varied in their effect; and 

 since they illustrate the action of all, as well as being vitally 

 connected with the health and disease of the human ani- 

 mal, we are justified in devoting part of our study of phy- 

 siology to the action of these plant forms. 



Experimental study of bacteria. — (See Ex. LXXII.) If 

 we prepare three test tubes by placing in one chopped hay 

 and water, in another some milk, and in a third some wheat 

 and water, and allow all three to stand uncovered for some 

 time in a warm room, we may observe the following changes 

 in the contents of the tubes : 



The hay solution becomes turbid, and a scum forms on its 

 Burface, while from the mixture comes a, disagreeable odor. 



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