226 



GENERAL SCIENCE 



comparative results of drinking unaltered and filtered 

 water in four of our American cities is shown graphically 

 in the diagram below. 



155. Methods of Purifying Water. (a) Boiling the 

 water will kill the germs in it but will not take out any 

 impurities. Water must be clean enough to drink before 

 boiling, except for a few living germs, or it will not be 

 fit for drinking after boiling. The destruction of germs 

 by boiling is practicable only in private homes. It 

 would be too expensive for the city water supply. 



M N M N M N M N 



BlNGHAMTON, N. Y. WATERTOWN, N. Y. ALBANY, N. Y. LAWRENCE, MASS. 



DIAGRAM SHOWING HOW SUPPLYING A CITY WITH GOOD WATER 

 LESSENS SICKNESS AND DEATH 



The lines M above each city show the relative number of persons dying 

 of typhoid fever before the water was filtered. The lines N show the num- 

 ber dying after the water was filtered. The figures are the number of deaths 

 from typhoid occurring out of 100,000 inhabitants. 



(b) Distilling is the most effective method of purifica- 

 tion. It removes from the water not only disease germs 

 but also many other impurities, such as dissolved salts. 

 Artificial ice is made of distilled water and hence it is 

 free from germs. But distillation would be a very ex- 

 pensive method of purifying water for cities. 



(c) Filtering with the aid of a few chemicals is the 

 method used for purifying city water. The filter bed is 



