1 86 



ELEMENTS OF PHYSIOLOGY 



disease, or the power of the blood to develop it, remains 

 and affords protection from the bacteria. 



322. Destruction of Bacteria outside of the Body. The 

 sun is the great destroyer of bacteria in summer and in 

 tropical climates. Cold destroys them in winter and in 

 cold climates. Sternberg has shown that exposure to one 

 hour of sunshine destroys the bacillus of consumption. 

 Wind drives away bacteria. It is almost impossible to 

 transmit disease in the open air. Some germs feed upon 



FIG. 160. Poorly Drained and Un 



healthful Site for House. 



a, sand; , clay. 



FIG. 161. Healthful House. 

 a, sand; b, rock. 



other germs. In the soil are certain kinds of germs which 

 oxidize all organic matter, including other kinds of germs. 

 Thus germs can filter through only a few feet of sand 

 before they are destroyed. Well water is safe to drink in 

 the country, if no impurities reach the soil near the well, 

 but in towns where the ground becomes soaked with germ- 

 laden sewage, impurities penetrate farther and well water 

 is unsafe (Figs. 162, 163). 



323.. Cleaning with soap and water removes germs. Dry- 

 ing kills many germs, but some species are harder to kill 

 when dry ; in that condition they will stand even freezing. 

 Heat, equal to that of boiling water, applied for 15 min- 

 utes, will kill almost every kind of germs. It does no 

 harm to leave air in the top of canned vegetables and jars 

 of preserves, but if they are to keep fresh, the germs must 

 be killed by a boiling temperature. Milk is a nourishing 





