128 Physiology. 



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., pre- 

 serve 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. If milk is heated to 

 1 60 or 170 F., any germs of tuberculosis present will be 

 killed. Boiled milk is less readily digested, and it is not 

 necessary to boil it to kill most kinds of germs that it may 

 contain. 



Summary. I . Dust as mere dry, dead matter is irritating. 



2. Disease germs may form part of the dust of the air. 



3. Most of our contagious diseases are known to be due to bacteria. 



4. Burning is the surest method of destroying germs. 



5. Carpets, tapestries, and cloth-upholstered furniture add largely 

 to the dust in houses. 



6. Putrefaction is caused by bacteria. 



7. Preservation of food depends on destroying, excluding, or retard- 

 ing the growth of the bacteria of putrefaction. 



Questions. I. Is the air in the mountains or on the seashore better 

 than elsewhere ? 



2. What regions are recommended for consumptives? Why? 



3. How is milk sterilized? 



4. Why do people seldom take cold while " camping out "? 



5. Why are the "steel grinders," in factories, short-lived? 



6. What occupations should be avoided by one who is predisposed 

 to consumption ? 



7. What are some of the occupations suitable to those predisposed 

 to consumption? 



