f 



CHEMICAL ANALYSIS 53 



The odour in the closed bottle increased ; in the second case 

 of exposure to air it was very slight after twenty-four hours, 

 and none after forty-eight hours. Under natural conditions, 

 therefore, the sewage would have become inoffensive. 



The open aeration test indicates approximately what takes 

 place in shallow or moving bodies of liquid. Dibdin and 

 Thudichum^ allow a mixture of i part of effluent and i of 

 fully aerated water to stand in an open vessel of diameter equal 

 to the depth, and estimate the dissolved O daily ; it should not 

 fall below 50 per cent, of saturation. When Thames water was 

 up to this limit they found no suggestion of nuisance, and fish 

 migrated freely. When, however, the large quantity of organ- 

 isms and organic matter take up oxygen more quickly than it 

 can be absorbed from the atmosphere the water becomes foul. 



I have examined this test very carefully,'^ as it is nearer to 

 ordinary conditions than closed incubation. The relation of 

 surface to volume must be uniform in different experiments. 

 Dibdin's suggestion of " diameter equal to the depth " is un- 

 satisfactory, since calculation will show that in that case the 

 surface-volume ratio varies with vessels of different diameters. 

 I recommend as the best directions for carrying out the test 

 uniformly " the samples should be kept at a uniform tempera- 

 ture in cyhnders 6 inches deep and 3 inches wide, exposed to 

 air and light, but protected from dust. The volume will then 

 be about 600 c.c, and the ratio of surface to volume i to 6." 



Even with uniform physical conditions the action varies 

 greatly with the nature of the organisms in the sewage, and in 

 the water used for dilution. A filtered water-supply is far less 

 active than river water. 



Distilled water and sewage, when sterilized and air-free, 

 absorb oxygen from the air at very nearly the same rate until 

 they are saturated, whereas with raw sewages the absorption is 

 at first normal. After a few hours the consumption of oxygen 

 overpowers the absorption from the air, so that the amount in 

 solution sinks considerably below 50 per cent, in twenty-four 

 hours. 



The closed incubator test made prominent at the Manchester 

 inquiry of 1899 is differently carried out : " A determination is 

 first made of the O absorbed from permanganate by the original 

 sample in three minutes. A bottle is then completely filled 

 with the sample, and closed and placed in the incubator at 



1 Journal of the Society of Chemical Industry, June 30, 1900. 



2 Analyst, xxvi., 305, August, 1901. 



