Effects of Physical Forces on Bacteria 163 



hydrated foods, the same precautions with respect to refrigeration 

 must be observed or the food will undergo spoilage the same as the 

 original product before the water was removed. Filterable viruses 

 are quite resistant to drying. Just as slow freezing is more destruc- 

 tive to bacteria than is fast freezing, we find that slow dehydra- 

 tion acts in a similar way. Lipman reported finding bacteria in 

 desiccated adobe bricks where he felt they might have remained in 

 suspended animation for centuries. 



The role of moisture in sterilization technics has been discussed 

 in a previous chapter. 



^-* '^ PRESSURE 



MECHANICAL 



At sea level the atmosphere exerts a pressure of 14.7 pounds 

 per square inch (one atmosphere), and with increases in eleva- 

 tion above sea level, this force is reduced as the air becomes 

 more rarefied. In recent years with the advent of stratosphere and 

 super-stratosphere flying in commercial airlines as well as in jet 

 planes, man has been forced to develop ingenious devices to allow 

 the occupants of these planes to survive not only the rarefied oxygen 

 but the tremendous changes in atmospheric pressure from those 

 found at sea level. One need not be a jet pilot, however, to ex- 

 perience in a mild way the effect of changing atmospheric pressure 

 on the mind and on the body. Transport yourself to the top of a 

 high mountain, such as Pike's Peak (elevation 14,109 feet), and 

 the atmospheric effects become only too apparent. The reduced 

 oxygen per unit volume of air makes some folks giddy, others 

 sleepy, and still others ill. Even mild exertion calls for materially 

 increased breathing. Water boils at about 187° F. at this elevation 

 and cooking foods like potatoes in an open vessel may require 

 twice the time that it does at lower elevations. 



Man can adapt himself to these atmospheric changes, and one 

 of the evident alterations he makes is in the number of oxygen- 

 carrying cells in the blood. A normal red blood cell count of five 

 million per cubic millimeter may rise to six million or more at 

 higher elevations. Since each cell can carry just so much oxygen,. 



