284 Scientific Proceedings, Royal Dublin Society. 
Then 
Since v is generally small in comparison with V, it may be 
neglected, except in cases where the utmost accuracy is desired. 
The equation then becomes— 
yo =20+ pte — p)- 
The reason for adjusting the level of the water at the zero of 
the scale at the top of the capillary portion of the tube aa, at the 
commencement of the experiment, will be understood from the 
last equation. It is evident from it that, if this were not done, a 
considerable absorption of oxygen would take place from the 
atmosphere of bottle B before the water in the tube aa would 
rise to the graduated portion of it. It may be here remarked 
that it is inconvenient to have the graduated portion of the 
tube aa inside the bottle A, owing to the difficulty of reading the 
scale when it is so placed. 
As regards the dimensions of the apparatus, these must be 
varied to meet the object for which it is required :—whether, 
for instance, it is required (1) to control the pollution of waterways 
within standards necessary for safeguarding the interests of 
public health, or of fish-life; (2) to determine the quality of 
an effluent from a continuous filter-bed or from a contact bed ; 
(3) to determine the strength of a sewage in a comparatively 
unpurified condition, e.g., tank-liquor. 
For the first purpose, an apparatus will be required that will 
accurately indicate very small volumes of oxygen absorbed in B. 
The volumes allowable under the standards formulated for the 
protection of fish-life must obviously be small; they will vary with 
the physical conditions of the stream, with such conditions as 
depth, flow, tidal or non-tidal, and distance from the open sea. 
Under the least favourable physical conditions, the allowable 
absorption may be taken to be about 5°d c.c. per litre of water at 
the evolution of nitrogen into the atmosphere of the bottle B, which might otherwise 
occur if the sewage were examined in the undiluted condition, since omnde sewage is 
generally supersaturated with free nitrogen. 
