BACTERIA IN WATER 51 



number condemns because it indicates probable pollution 

 with surface water or sewage in order to supply pabulum 

 for so many bacteria per cc. 



It should always be remembered that a chemical report 

 and a bacteriological report should assist each other. The 

 former is able to tell us the quantity of salts and condition 

 of the organic matter present ; the latter the number and 

 quality of micro-organisms. Neither can take the place of 

 the other and, generally speaking, both are more or less use- 

 less until we can learn, by inspection and investigation of the 

 source of the water, the origin of the organic matter or con- 

 tamination. Hence a water report should contain not only 

 a record of physical characters, of chemical constituents, and 

 of the presence or absence of micro-organisms, injurious and 

 otherwise, but it should also contain information obtained by 

 personal investigation of the source. Only thus can a reason- 

 able opinion be expected. Moreover, it is generally only 

 possible to form an accurate judgment of a water from 

 watching its history, that is, not from one examination only, 

 but from a series of observations. A water yielding a steady 

 standard of bacterial contents is a much more satisfactory 

 water, from every point of view, than one which is unstable, 

 one month possessing 500 bacteria per cc. and another 

 month 5000. It is obvious that rainfall and drought, soil 

 and trade effluents, will have their influence in materially 

 affecting the bacterial condition of a water. 



It is perhaps scarcely necessary to add that we have not 

 in the above account of the examination of water included 

 all, or nearly all, the various methods adopted for acquiring 

 a knowledge of the bacterial contents of the water. Many of 

 these are of too detailed and technical a nature to enter into 

 here. Three points, however, we may touch upon. In the 

 first place, as we have said, the particulate matter out of a 

 large body of water should be concentrated in a small 

 quantity. Accordingly it has become the custom to pass 



