BACTERIA IN WATER AND MILK. 109 



The ordinary soil bacteria in water are harmless. 

 It is only the pathogenic bacteria in the soil from 

 human excreta, like the typhoid and dysentery bacilli 

 and the cholera spirilla, that get into the water and 

 cause disease. In testing the water to see whether it 

 can transmit these diseases or not, it is almost useless 

 to look for the disease-producing bacteria themselves, 

 because they are extremely difficult to find. The pres- 

 ence of intestinal bacteria is looked for, particularly 

 the colon bacillus, and when they are found in large 

 numbers the water is condemned for drinking pur- 

 poses: first, because drinking-water should not con- 

 tain substances excreted from the intestines of man or 

 animals, and, secondly, water that does contain such 

 substances is constantly open to infection with bacteria 

 that produce disease. 



Nowadays practically all surface waters are con- Artificial 



meth ds of 



tammated with human sewage. To render these purification 

 waters safe for drinking purposes in cities, the natural 

 process of water purification cannot be relied upon, and 

 artificial methods, based on filtration, are employed. 

 The water may be made to percolate through beds 

 made of fine gravel and covered with a thick layer of 

 fine sand. The dirt and slime in the water cling to 

 the small particles of the sand, and only the water free 

 from its impurities is permitted to pass through. 

 About 90 per cent, of the bacteria in water can be 

 removed by sand filtration. In mechanical filtration, 

 a chemical substance like alum is added to the water 

 in sufficient quantity to coagulate the solid and ex- 



