48 Elements of Water Bacteriology. 



£nger or the entrance of a particle of dust may wholly 

 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 iil 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. DifiEerent observers 

 obtained inconsistent results, and Bolton (Bolton, 1886) 

 concluded that there was no relation whatever between 

 the chemical composition of a water and its bacterial con- 

 tent. Tiemann and Gartner (Tiemann and Gartner, 

 1889) furnished the key to the difficulty in their state- 

 ment that there are two classes of bacteria, the great 

 majority of species, normally occurring in the earth 

 or in decomposing organic matter, which require abun- 

 dance of nutriment, and certain pecuUar water bacteria 

 which can multiply in the presence of such minute 

 traces of ammonia as are present in ordinary distilled 

 water. Even these prototrophic or semi-prototrophic 

 forms require a definite amount of food of their own 

 kind. 



