DUST, MOLDS, AND BACTERIA OF THE AIR 95 



sterilization. The decay of substances may also be hindered 

 through drying or refrigeration. Many foods, such as apples, 

 grapes, plums, peas, and beans, may be kept for a long time 

 if they are dried and kept in a dry place. For other fruits 

 and vegetables the skin serves as sufficient protection for a 

 long time if they are kept in a cool, dry place where there 

 is a good circulation of air. These measures protect against 

 the growth of mold. 



107. Bacteria and disease. Bacteria that are able to grow 

 parasitically in the bodies of men, animals, and plants fre- 

 quently produce more or less serious diseases. In many cases 

 the bacteria are easily transferred from one person to another, 

 and such diseases are said to be infectious. Other bacteria of 

 this class are distributed through the agency of air or water. 



Examples of bacterial diseases are boils and carbuncles, 

 blood poisoning, lockjaw, pneumonia, diphtheria, typhoid fever, 

 influenza, tuberculosis, leprosy, cholera, and the plague. 

 Recently we have come to realize that common colds are due 

 to bacteria and are infectious. We should be careful, there- 

 fore, not to spread colds by coughing and sneezing. 



108. Useful bacteria. Bacteria are of direct use in many 

 manufacturing processes, for instance, in vinegar-making, for 

 the fermentation that produces the sour substance in vinegar 

 is the result of bacterial action. Others of these organisms 

 cause the change in cream known as "ripening," and the 

 flavor of butter is due, in part at least, to this kind of bac- 

 teria. In cheese-making the work of bacteria is indispensa- 

 ble, and a large part of the difference in the flavor of the 

 different varieties is due to the different kinds of bacteria 

 which have assisted in the ripening, though some kinds of 

 cheese are ripened by certain molds. 



109. Useful yeast. One of the most useful dependent 

 plants is the yeast which is used for bread-making. The 

 yeast feeds on the sugar in the dough and produces alcohol 

 and carbon dioxide. The carbon dioxide is prevented from 



