190 BACTERIA IN AIR, SOIL, WATER, AND MILK 



not travel far by this means if air currents be absent, 

 and there is probably little danger from simple unpleas- 

 ant odors from drains if protected from these currents. 

 Bacteria are found in air by filtering a definite quantity 

 of air through cotton or sugar. The former is washed, 

 the latter dissolved in sterile water, and this is examined 

 as outlined under water. 



BACTERIA IN SOIL 



Microorganisms live in the first few feet of the earth 

 wherever moisture and a small amount of nourishment 

 are found. Pathogenic forms are nearer the surface, 

 while deeper in pure saprophytes are found. Bacteria 

 are deposited from dust, water, and the dejecta of 

 animals. Pathogenic ones are only to be found where 

 animal life exists, while in uninhabited or untilled 

 lands they probably do not exist at all. The denser 

 the population the more disease-producing kinds are 

 found. Rain will wash away soil and carry with it 

 bacteria into water-courses. This is of great impor- 

 tance where human dejecta are deposited on the 

 ground. 



Typhoid fever epidemics have been known to have 

 such an origin. Typhoid germs can live in soil for many 

 months. Vegetables are easily contaminated, and if 

 eaten raw can transmit the disease. Cholera vibrios 

 have a shorter life in this place. Anthrax bacilli live 

 a long time and cattle are sometimes infected by 

 their pasture. Actinomycosis is well known to spread 

 through a herd because of infected pasture land. 

 The bacteria of soil are found by planting some of it 



