43 BACTERIA IN WATER 



cent, in excess of the average, kept the colour and amount of soluble vegetable 

 matter in solution abnormally high. In December, owing to the continued rains, the 

 New River and Thames unfiltered waters contained a maximum number of bacteria. 

 In dry weather the number per c.c. had been as low as 149 and 2013 respectively, 

 but owing to seasonal changes they had risen to 861 and 27,216 bacteria per c.c. 

 respectively. 



From these various records we find that in the result the number 

 of bacteria in river water depends upon a variety of circumstances, 

 amongst which the most important direct conditions are four, namely, 

 (1) local pollution, (2) natural purification (to which subsequent 

 reference will be made), (3) season and rainfall, and (4) sedimentation 

 and filtration. Behind these direct conditions we have also seen that 

 time, temperature, light, exposure to air, and the presence of organic 

 matter play an essential part. 



Bacteriological Examination of Water. [See Appendix, p. 463.] 



Quantitative Standard. In arriving at a conclusion respecting the 

 number of organisms in a water and their bearing upon its suitability 

 for use, it should be remembered that a chemical report and a 

 bacteriological report are desirable before forming an opinion. 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 useless until we can learn, by inspec- 

 tion and investigation of the source of the water, the origin of the 

 organic matter or contamination. Hence a water report should con- 

 tain not only a record of physical and microscopical characters, of 

 chemical constituents, and of the presence or absence of micro- 

 organisms, injurious and otherwise, but it should also contain infor- 

 mation obtained by personal investigation of the source. Only thus 

 can a reasonable opinion be expected. Moreover, it is generally only 

 possible to form an accurate judgment of a water by watching its 

 history ; that is to say, not from one examination only, but from a 

 series of observations. The writer has examined a certain water 

 supply for thirty-six consecutive months. In 1901 the average 

 number of bacteria per c.c. was 93, in 1902, 136, and in 1903, 57. 

 This shows a stable bacterial content which in itself is favourable. 

 A water yielding a steady standard of bacterial content is a much 

 more satisfactory water, from every point of view, than one which 

 is unstable, one month possessing 50 bacteria per c.c. and another 

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

 effluents, time and temperature, will have their influence in materially 

 affecting the bacterial condition of a water. 



Miquel and others have suggested standards which allow " very 

 pure water" to contain up to 100 micro-organisms per c.c. Pure 

 water must not contain more than 1000, and water containing up to 



