QUANTITATIVE BACTERIOLOGICAL ANALYSIS 53 



destroy the accuracy of an examination. Even the 

 slight disturbance of conditions incident upon the 

 storage of a sample after it has been taken may in a 

 few hours wholly alter the relations of the contained 

 microbic life. It is necessary, then, in the first place, 

 to exercise the greatest care in allowing for possible 

 error in the collection and handling of bacteriological 

 samples; and in the second place, only well-marked 

 differences in numbers should be considered significant. 



In the early days of the science, discussion ran high 

 as to the interpretation of bacteriological analysis; 

 and particularly as to the relation of bacterial numbers 

 to the organic matter present in a water. Different 

 observers obtained inconsistent results, and Bolton 

 (Bolton, 1886) concluded that there was no relation 

 whatever between the organic pollution of a water and 

 its bacterial content. Tiemann and Gartner (Tiemann 

 and Gartner, 1889) furnished the key to the difficulty 

 in their statement that there are two classes of bacteria, 

 the great majority of species normally occurring in 

 the earth or in decomposing organic matter, which 

 require abundance of nutriment, and certain peculiar 

 water bacteria which can multiply in the presence of 

 such minute traces of ammonia as are present in ordi- 

 nary distilled water. Even these prototrophic or 

 semi-prototrophic forms, however, require a definite 

 amount of food of their own kind. 



Kohn (1906) determined the minimal nutrient mate- 

 rial requisite for certain of them and found that they 

 could develop in the presence of 198 X io~ 10 to I98X io" 13 

 per cent of dextrose, 66Xio~ 13 to 66Xio" 17 per cent 



