DUST AND BACTERIA. 127 



The Bacteria of Putrefaction. Besides the disease- 

 producing bacteria, there are others that cause decay and 

 putrefaction of various kinds. They cause our richer foods 

 to " spoil," milk to turn sour, butter to become rancid, etc. 



While these bacteria do not cause disease in the human 

 body, they often make food poisonous. The cases fre- 

 quently reported of poisoning from eating ice cream, 

 cheese, sausage, etc., are in many cases due to bacteria in 

 them. We should, in the first place, be careful to get 

 good, fresh material. In the second place, it should be so 

 kept as to prevent the introduction and development of 

 bacteria in it. Bacteria need heat for their growth (as we 

 so well know is the case with the higher plants). They 

 also need moisture. 



The Preservation of Foods. So our principal modes 

 of keeping foods from spoiling are to keep them in a cold 

 place, or to dry them. Or we heat them, and shut them 

 away from the air, as in our various modes of canning and 

 preserving foods. Salting and smoking meats, etc., preserve 

 them by preventing the growth of bacteria. Cold does 

 not usually kill bacteria. So milk that has been kept in a 

 refrigerator, and that seems sweet, may have in it a stock 

 of bacteria, and after we drink the milk the heat of our 

 bodies favors their development. There are now known 

 ways of killing the bacteria in milk and other liquids, known 

 as " sterilizing," that make us safe from this danger. 



Although the main subject of this chapter is air and 

 ventilation, it has been thought best to touch briefly the 

 subject of bacteria in food, as the bacteria are so widely 

 disseminated by the air. One of the earlier and still in- 

 teresting works on this subject is Tyndall's Floating Matter 

 of the Air. 



