THE HUMAN BODY. 



carried through lead pipes, it sometimes dissolves enough 

 of the lead to become poisonous. Whether it will do this, 

 or not, depends on what it already contains. Some 

 waters may be carried through lead pipes with perfect 

 safety, either because the water does not act on the lead 

 at all, or because it contains certain mineral matters 

 which form a crust, lining the pipes, and protecting 

 them from further action. When lead pipes are used, 

 the question whether the water acts upon them, should 

 be settled by a chemist if necessary. Tin pipes are safer. 

 Water that is constantly running through lead pipes is 

 less likely to contain much lead than that which stands 

 still in the pipes for long periods without change. Lead, 

 when taken in small doses in this way, produces its 

 effects very gradually ; and the health is often seriously 

 affected before the cause is discovered. Among the symp- 

 toms of lead-poisoning are colic, and paralysis of certain 

 muscles. 



34. Offensive and poisonous matters, animal and vege- 

 table, sometimes find their way into drinking-water, and, 

 being dissolved in it, give no sign of their presence. 

 Sewage, and the germs of disease, may thus be taken in. 

 It is necessary, therefore, to guard the well, or water-pipes, 

 very carefully from all impurities. Wells and reservoirs 

 are often placed where they catch the drainage from barn- 

 yards, or other receptacles for filth. Such drainage will 

 go through the soil much farther than is commonly sup- 

 posed. The fact that water is clear and sparkling and 

 odorless does not prove it pure. A well or reservoir 

 should not be located within thirty feet of any filthy spot. 

 Even at that distance it is not safe if the ground is porous, 

 and slopes toward the well. 



