PHYSICAL AGENTS 



143 



worms, or movement of ground water to the surface. Also 

 that the underground water supply which is drawn upon 

 for use by men or animals is not also contaminated. Fre- 



FiG. 106. — Berkefeld filter candles about one-half natural size. 



quently infected material, carcasses of animals, etc., are 

 treated in some way so as to aid the natural process of 

 destruction of the organisms present, especially by the use 

 of certain chemical agents, as quicklime (seep. 146). 



