EXAMINATION OF WATER 295 



of bacteria, but the number contained therein varies very 

 much with the source of the water. In stagnant water, 

 such as is found in brooks and small ponds, the number 

 of micro-organisms present is always very great. The 

 comparatively pure water of large lakes and upland streams 

 often contains many bacteria, but these are always harm- 

 less saprophytes, which find their normal habitat in such 

 waters. Thus, in the purest upland streams and lakes 

 we frequently find that the number of bacteria in 1 c.c. is 

 under 100, while in town sewage are many millions in the 

 same volume. In ordinary rivers the number is generally 

 between 1,000 and 100,000 per c.c. In the case of waters 

 from deep-seated springs, the presence of more than 100 

 organisms per c.c. is conclusive evidence that the water 

 has undergone some contamination with surface-water. 

 Micro-organisms are also found, although not in great 

 numbers, in rain-water, hail, snow, and even in the ice 

 of glaciers. The water-supplies of large towns come for 

 the most part either from rivers or lakes, with supple- 

 mentary supplies from wells. Many of these water-supplies 

 under ordinary circumstances may be satisfactory, but there 

 is always the danger of sewage-pollution. It must not be 

 forgotten that all polluted waters have a natural tendency 

 to purify themselves if exposed to the air. As regards the 

 nature of the organisms found in natural waters, they are 

 for the most part bacilli, micrococci being somewhat rare, 

 while spirilla are not unfrequently found. About 240 

 species have already been discovered in water, the majority 

 being harmless saprophytes, although many pathogenic 

 species are also found. As has already been pointed out, 

 typhoid and cholera are essentially water-borne diseases. 



In many of the recorded cases of water-borne typhoid 

 and cholera, the amount of organic matter accompanying 

 the specific pollution was so extremely small that the water- 



