48 SCIENCE OF HOME AND COMMUNITY 



2. To test for animal impurities. To the sample of water 

 to be tested add a few drops of nitric acid and a few drops of 

 silver nitrate and stir. If the water becomes milky, animal 

 impurities are present. This is not an infallible test, because 

 the presence of chlorides in the water will also give a white 

 precipitate. 



Bored wells. Sometimes bored wells are used. This is 

 really a convenient way of digging a small well. Soil augers 

 are used. The soil is removed for a considerable depth 

 and the hole lined with tiling. The same precautions should 

 be observed with reference to this well as for the dug well. 



Driven wells. To make a driven well, a pipe with a hard 

 point is driven into the ground till a layer of soil is reached 

 that has a constant supply of water. The pipe is removed 

 and the driving point is replaced with a filtering point covered 

 with a sieve, which allows the water to enter but prevents 

 the dirt from filling the pipe. The pipe is then driven down 

 again. Under favorable conditions these wells may prove 

 satisfactory. The chief objections are that the amount of 

 water that can be pumped at one time is small and that the 

 pipe may get clogged with dirt, especially if the soil is very 

 fine. 



Drilled wells. On the whole the drilled well is the most 

 satisfactory. By means of drills and machines, a hole is 

 drilled in the ground and a pipe driven down the hole as 

 fast as it is drilled. Wells of this type may be drilled to a 

 great depth, and the water thus obtained is pure, if care is 

 taken that the pipes do not leak. 



Springs. Occasionally springs are so located that they 

 may furnish a satisfactory source of water. The chief con- 

 sideration here, as with dug wells, is the cleanliness of the 

 surroundings. If there is no source of impurity near, the 

 spring will furnish a supply of good water. But care should 

 be taken to prevent surface pollution. It is well to build up a 

 casing about a foot above the ground the same as with a well. 



