THE PUBLIC WATER, MILK, AND FOOD SUPPLY 351 



break, one was sure to follow in Lawrence. People soon 

 learned that the reason was that the bacteria from the sick 

 people in Lowell were carried down the river into the drink- 

 ing water of Lawrence. After a careful study of the situa- 

 tion, the people of Lawrence decided to put in large outdoor 

 niters in order that the river water might pass through these 



FIG. 145. Cases of typhoid per 100,000 inhabitants before filtering water supply 

 (solid) and after (shaded) in A, Watertown, N. Y. ; B, Albany, N. Y.; C, Law- 

 rence, Mass. ; D. Cincinnati, Ohio. 



and be purified before it was used by the people of the city. 

 These filters worked so well in taking out the typhoid germs 

 that only one fifth as many people died each year from 

 typhoid fever as had formerly (see figure 145), and when the 

 next epidemic of the disease appeared in Lowell, Lawrence 

 was not affected. 



DEMONSTRATION 27 



Purpose. To see which kinds of water contain the fewest 

 bacteria. 



Materials. Sterilized test tubes containing culture medium, 

 pipette, petri dishes. 



Directions, i. The culture medium can be bought all pre- 

 pared from dealers directly, or through drug stores. It can 

 also be made as explained in Conn's Bacteria, Yeasts and 



