PURIFICATION OF WATER 169 



and the washing of meats and of many other foods will have 

 some influence in retarding decomposition changes. 



Purification of water. Water may be classed as a food ; 

 and, while it is not subject to decomposition, and hence one 

 cannot speak of preserving it, it does serve to distribute 

 disease-producing organisms. The measures that are taken 

 to preserve foods often serve to destroy any pathogenic or- 

 ganisms they may contain, just as they destroy the sapro- 

 phytic organisms. 



Bacteria, harmful and otherwise, may be removed from 

 water by passing it through filters of unglazed porcelain. 

 The pores of these filters are very fine and tortuous, so that 

 the bacteria are held back even though the pore spaces are 

 of greater average diameter than the organisms. If, how- 

 ever, the filters are used for a considerable period of time 

 without cleaning, some forms of bacteria will multiply in 

 the pores and the filtrate will no longer be sterile. The 

 filters must be frequently cleaned and sterilized, if they are 

 to function in an efficient manner. The purification of 

 water in percolating through the soil is also due to the same 

 principle. 



Many cities purify their water supplies by filtering sur- 

 face waters through filters composed of layers of gravel 

 and sand, arranged so that the finer material is on the 

 surface. The upper surface of the filter soon becomes 

 coated with a gelatinous layer of bacteria and sediment 

 which is so coherent that it prevents the organisms in the 

 water from passing through the sand. As this sediment 

 layer increases in thickness, it becomes less permeable, 

 thus reducing the amount of water that will pass the filter. 

 In time this filtering surface must be removed. For the 

 first few days after this disturbance, the efficiency of the 

 nitration process is reduced, until the gelatinous layer is 

 reestablished. By means of this process, it is possible to 



