DRINKING WATER 137 



the substances themselves have become so far decayed that 

 they dissolve like salt and leave the water clear, and odor- 

 less, and tasteless. 



221. Source of impurities. The usual way in which 

 disease germs reach drinking water is by means of sew- 

 age, and slops, and barnyard drainage. There is hardly 

 a house in which, during a year's time, some sick per- 

 son does not give off disease germs. The germs are 

 found in all the waste matters which come from the sick 

 person's body and in the dish water and bath slops from 

 the house. The germs may live and grow in the slops 

 after they have been emptied, and may find their way into 

 a well, and thus may reach any one who drinks the water. 

 They may be washed into a river and cause sickness 

 among those who use its water or ice. This is the usual 

 way in which typhoid fever is spread. The number of 

 typhoid fever cases in a town is often taken as an indica- 

 tion of the purity of the water supply. By keeping the 

 water supply pure this disease may be almost suppressed. 

 It might be entirely suppressed by disposing of all sewage 

 in a harmless way (see pp. 251-253). 



222. Purification by oxidation in the soil. The ground has 

 the power of oxidizing decayed vegetable and animal matter so that 

 only the mineral parts remain. Slops from the house are thus oxidized, 

 if the ground is not soaked through with them. But when the quantity 

 is great, some may work their way through the ground for a considerable 

 distance and finally enter a well. 



223. Purification by filtration. Clean sand has the power to 

 screen out particles carried by the water. Screening out substances 

 from water by passing it through a powdered substance is called filtra- 

 tion. As the slops slowly soak through the soil, their solid parts are 

 filtered out in the first few inches of the top soil, and if the quantity is 

 not too great, are soon oxidized. Soils differ in their ability to filter. 

 Clean sand is the best ; clay is the poorest. It is almost impossible to 



