XXI. BACTERIOLOGICAL ANALYSES. 



EACH bacteriological or bacterioscopical analysis of 

 air, earth, sewage, various food-stuffs, etc., includes, 

 as a general rule, two distinct investigations yielding 

 results of very unequal value : 



1. Quantitative. 



2. Qualitative. 



The first is purely quantitative and as such is of 

 minor importance as it aims simply at enumerating 

 (approximately) the total number of bacteria present 

 in any given unit of volume irrespective of the nature 

 and character of individual organisms. 



The second and more important is both qualitative 

 and quantitative in character since it seeks to accurately 

 identify such pathogenic bacteria as may be present 

 while, incidentally, the methods advocated are calcu- 

 lated to indicate, with a fair degree of accuracy, the 

 numerical frequency of such bacteria, in the sample 

 under examination. 



The general principles underlying the bacteriological 

 analyses of water, sewage, air and dust, soil, milk, ice 

 cream, meat, and other tinned stuffs, as exemplified 

 by the methods used by the author, are indicated 

 in the following pages, together with the methods of 

 testing filters and chemical germicides; and the tech- 

 nique there set out will be found to be capable of expan- 

 sionand adaptation to any circumstance or set of cir- 

 cumstances which may confront the student. 



Controls. The necessity for the existence of ade- 

 quate controls in all experimental work cannot be 

 too urgently insisted upon. Every batch of plates that 

 is poured should include at least one of the presum- 

 es 



