QUANTITATIVE BACTERIOLOGICAL ANALYSIS. 37 



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 in 

 the collection and the handling of bacteriological samples; 

 and in the second place, only well-marked differences in 

 numbers should be considered as possibly significant. 



In the early days of the science, discussion ran high as 

 to the interpretation of bacteriological analysis; and par- 

 ticularly 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 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 great 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 peculiar water bacteria 

 which can multiply in the presence of such minute traces 

 of ammonia as are present in ordinary distilled water. 

 Bacteria of the second class under abnormal conditions, 

 as in bottled samples, or, at times, in a well or the basin of 

 a spring, may occur in great numbers where there is but 

 little organic matter. In normal surface-waters, how- 

 ever, such growths have not been recorded as far as we 



