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"These two types of test illustrate two distinct points of view 

 which slioiild be clearly differentiated. An effluent may be regarded 

 as being composed of a given mass of organic matter dissolved or 

 suspended in a definite amount of water. The water contains also 

 a definite amount of available oxygen in the form of free dissolved 

 oxvgen, nitrites, nitrates, and possibly of other compounds. All 

 the organic matter is oxidizable to some extent, and to that extent 

 it serves as bacterial food. The greater the amount of organic mat- 

 ter and the greater its oxidizability, the greater is the absorption of 

 oxygen from the medium. Consequently a reduction of available 

 oxygen in the effluent during incubation is a measure both of the 

 amount of organic matter present and of its capability of oxidation. 

 As a small amount of readily oxidizable matter has the same effect 

 on the result as a larger amount of more stable matter, a test of this 

 kind indicates whether or not the organic matter consumes oxygen ; 

 but it does not show whether or not the supply of available oxygen 

 is sufficient to prevent the establishment of anaerobic conditions. 

 This important question of the balance between the oxygen demanded 

 by the organic matter and the oxygen available in the liquid is taken 

 into consideration by tests of the second kind mentioned, namely, 

 those dependent on the establishment of anaerobic conditions. Such 

 tests do not involve estimation of the amount and the kind of 

 organic matter; indeed, organic matter which does not absorb any 

 oxygen from the liquid under the conditions of an incubation test 

 must be very highly oxidized ; and, furthermore, most organic mat- 

 ter derived from sewage is putrescible in itself — that is, if it is stored 

 by itself in the absence of oxygen, it undergoes putrefactive changes. 

 The question at issue is not, however, whether the organic matter 

 itself will putrefy, but whether the effluent as a whole will become so 

 reduced in oxygen that putrefaction will become possible. In other 

 words, it is simply a question of a balance between the available 

 oxygen of the effluent and the oxygen which the organic matter will re- 

 quire during the incubation period. It would seem that the problem 

 might readily be solved by determining this balance, but, unfortunately, 

 it is not a simple matter, because the action involved is bacterial. 

 Many attempts have been made to determine the oxygen balance ana- 

 lytically, but such tests answer only with very good and very bad 

 effluents, for which an inspection of the sample would serve just as 

 well. When there is doubt about the character of the effluent — the 

 condition for which such information is of most value — all such ana- 

 lytical procedures have heretofore failed. It is evidently impossible 

 to imitate with any degree of precision the bacterial activities that 



