Adeney — Dissolved Gases and Fermentative Changes. 567 



In the solution 1/40 the proportion of sewage-matters to dissolved oxygen was 

 evidently pushed to its maximum. That is to say, the dissolved oxygen present 

 was only just sufficient for the completion of the first stage of the fermentation of 

 the organic matters, viz. the breaking down of the organic matters into carbon 

 dioxide, water, and ammonia. 



Hence, from the results of these experiments, if the fermentation of sewage 

 matters really takes place in the two stages already referred to, we may tenta- 

 tively conclude that when the dissolved oxygen is present in sufficient quantity 

 for the completion of both stages, the quantities of carbon dioxide and nitric acid 

 formed, and the volume of the dissolved oxygen absorbed, are directly propor- 

 tional to the quantity of sewage-matters originally present. 



If, however, the oxygen be not in sufficient quantity for the comj^letion of 

 both stages, but that of the first only, then the volume of carbon dioxide formed 

 is still directly proportional to the quantities of sewage-matters originally present, 

 and the oxidation of ammonia will or will not proceed according as there happens 

 to be an excess or not of oxygen over and above that required for the first stage 

 of fermentation. 



In the solutions 1/30, 1/20, and 1/10 we have an insufficient quantity of dissolved 

 oxygen for the completion of the first stage of fermentation. We then see a 

 secondary, or an aerobic, fermentation sets in, in which the fermentation of the 

 organic matter has been proportionally less and less complete as the quantity 

 of sewage-matter increased. No simple relation between the carbon dioxide 

 formed, and the quantity of sewage-matter originally present is apparent under 

 this condition. 



We may go further, and sum up the results given in the two Tables in a 

 definite quantitative form, thus : — 1000 volumes of the sample of sewage-water 

 experimented with required 580 volumes of atmospheric oxygen, with a probable 

 error of 15 volumes, for the complete fermentation of the fermentable matters, 

 280 volumes of which were required for the completion of the first stage, and 300 

 volumes for the second stage of fermentation. Or we may put it in another way, 

 and state that when 1 volume of the sewage-water was mixed with 39 volumes of 

 good river-water, fully aerated and fairly pure, at winter temperature, the oxygen 

 dissolved in the river- water was sufficient for the first stage of fermentation ; and, 

 when mixed with 69 volumes, the supply of dissolved oxygen was suflScient for 

 both stages of fermentation of the admixed sewage-matters. The atmospheric 

 nitrogen determination for the solutions 1/100 to 1/40 inclusive, show greater differ- 

 ences than will be found usual. These may possibly not be due to experimental 

 errors, but rather to "fixation" of atmospheric nitrogen during fermentation 

 (see p. 587). 



