NUMERICAL STANDARDS 43 



100,000 bacteria per o.c. is contaminated with surface water or 

 sewage. Macd gives the following table: — 



Bacteria per c.c. 

 Very pure water . . to 50 



Good water . . 50 ,, 500 



Passable (mediocre) water 500 „ 3,000 



Bad water. . . ■3,000 ,, 10,000 



Very bad water . . 10,000 „ 100,000 and over. 



Koch first laid emphasis on the quantity of bacteria present as an 

 index of pollution, and whilst different authorities have all agreed 

 that there is a necessary quantitative limit, it has been impossible to 

 arrive at a settled standard of permissible impurity. Besson adopts 

 the standard suggested by Miquel, and on the whole French 

 bacteriologists follow suit. They also agree with him, generally 

 speaking, in not placing much emphasis upon the numerical estima- 

 tion of bacteria in water. In Germany and England it is the custom 

 to adopt a stricter limit. Koch in 1893 suggested 100 bacteria 

 per e.c. as the maximum number of bacteria which should be present 

 in a properly filtered water. Miquel holds that not more than ten 

 different species of bacteria should be present in a drinking water, 

 and such is a useful standard. The presence of many rapidly 

 liquefying bacteria or organisms associated with sewage or surface 

 pollution would, even though present in fewer numbers than a . 

 standard, condemn a water. From a consideration of all the facts 

 it will be seen that it is impossible to judge alone by the numbers. 

 As the science of bacteriology advances less emphasis is laid 

 upon quantitative estimation, for the reason that it is impossible 

 to gauge the quahty of a water only by such estimation. The 

 character of the organisms present and the relative abundance of 

 each species is of more importance than quantitative estimations. 

 Such estimations of water bacteria, based upon the counting of 

 colonies in plate cultures, are of little value, and are in no sense 

 an adequate bacteriological examination of a water. It is such 

 "examinations" which have brought bacteriology into disrepute, 

 for it is certain that estimations of this kind are frequently not even 

 approximately correct, nor do they furnish any final indication as to 

 safety or otherwise of a water supply. At the same time it should 

 not be forgotten that, other things ieing equal and constant, a low 

 number of organisms tends to indicate that a water has not been 

 contaminated with organic matter or the addition of foreign bacteria, 

 and has not been in a condition to favour multiphcation of bacteria, 

 and vice versd. Broadly speaking, it must be true that a water 

 containing a large degree of organic matter, the pabulum of bacteria, 

 will contain a higher number of bacteria than a water containing a 



