152 PHYSICAL INFLUENCES. 



are usually guaranteed only for a certain length of time, from one to 

 twelve months. The advantage of the dry culture is its better keeping 

 qualities. Liquid cultures produce substances harmful to themselves, 

 and die rapidly after a short time, while the dry cultures show little change. 

 The resistance of pathogenic bacteria to desiccation is of considerable 

 importance in the spreading of contagious diseases. Many pathogenic 

 bacteria die after desiccation of a few hours to a few days, and spreading 

 of such diseases by dust is highly improbable. Protozoa of soil decrease 

 in number by drying, but all are not killed. 



