74 PRACTICAL ZOOLOGY 



can be estimated by any one who is mathematically inclined. 

 Bacteria, like other living things, must have food (which con- 

 sists of mineral matter and plant or animal substances), mois- 

 ture, and usually free oxygen. Fortunately, only a few bacteria 

 cause diseases; these are called pathogenic. The others, or non- 

 pathogenic bacteria, act as scavengers in the earth, furnish food 

 for plants, and are used in various industries, such as in the manu- 

 facture of linen and cheese. Bacteria are almost everywhere; 

 they are in every breath of air we breathe, in most of the water 

 we drink, and are abundant in the earth. 



i 



D 



Fig, 48. — Disease germs — bacteria. 



A, typhoid; B, tuberculosis; C, cholera; D, anthrax; E, a blood corpuscle 



engulfing a germ. 



How to Destroy Bacteria. — It is necessary to know how 

 to destroy or prevent the increase of bacteria in order to protect 

 ourselves from the pathogenic kinds. Cleanliness will of course 

 dispose of most of those that rest upon the surfaces of our bodies. 

 Within the body, juices and white blood corpuscles are contin- 

 ually at work destroying many of those that manage to get in. 

 Various agents are employed for controlling bacteria. Sub- 

 stances called antiseptics prevent their growth ; disinfectants or 

 germicides kill pathogenic forms; and all kinds are killed by 



