234 EXPERIMENTAL GENERAL SCIENCE 



surest way of destroying bacteria, however, is to subject them 

 to great heat, either by boiling or steaming them. Even 

 temperatures much below the boiling point have been found 

 to be effective in the case of many kinds. Milk is pasteurized 

 by being brought to a temperature of 130F. and held at that 

 temperature for a few minutes. Bacteria are also rapidly 

 destroyed if exposed to the ultra-violet rays, and chemicals 

 of various kinds are also effective. Rendering objects free 

 from bacteria is called sterilizing. Chemicals used in killing 

 bacteria are usually called disinfectants. In the human 

 body, however, the problem of destroying bacteria is com- 

 plicated by the necessity of killing the bacteria without injur- 

 ing the cells. Substances which will accomplish this are 

 usually called antiseptics. Among substances used externally 

 for such purposes are tincture of iodine, boracic acid, carbolic 

 acid, hydrogen peroxide, bi-chloride of mercury, and alcohol. 

 Salt, sugar, and other substances, by extracting the water 

 from bacteria, affect them somewhat as ordinary drying 

 does. Such substances are often called preservatives. 



Practical Exercises 

 1. Give a reason for washing the hands before eating. 



2. Why is it desirable to wash thoroughly food that has been exposed 

 for sale in the open market? 



3. Why are cooked foods likely to be more wholesome than raw foods? 



4. Why might tea or coffee made from a suspected water be less 

 dangerous than the water itself? 



6. Why should refrigerators and other receptacles for food be fre- 

 quently cleaned? 



