542 Adeney — Dissolved Gases and Fermentative Changes. 



dioxide and possibly ammonia in the one; and of carbonic dioxide (if any be 

 formed) and nitrous and nitric acids in the other, together with accurate deter- 

 minations of tlie free oxygen consumed during both stages, would afford data for 

 estimating the actual quantity of fermentable matters present in polluted water 

 with sufficient accuracy for the purpose of water analysis. 



Such a metliod of examination as here suggested promised results of a most 

 definite character, and of especial value for the examination of polluted drainage 

 waters, in that they would probably afford the means of accurately estimating the 

 quantity of such water which could be safely allowed to drain into a river or stream 

 of a given size, and would thereby lead to the possibility of formulating exact 

 practical standards of impurity for drainage waters, which standards could be 

 graduated according to local conditions of volume and flow of the stream. 



The method of examination which I have sketched was, however, open to 

 possible objections, which could only be answered one way or the other by a full 

 experimental investigation. For instance, a water under examination would 

 always contain mixed organisms, and the selective chemical functions of these 

 might vary to such an extent that in no two experiments with the same water 

 would similar values for the products formed, or oxygen consumed, be obtained. 

 It might further be objected that a given result was due to the accidental pre- 

 sence of a particular organism. 



There were, however, good reasons for believing that, if precautions were taken 

 to experiment under certain conditions, as regards the relative quantities of 

 fermentable matters, and of dissolved oxygen, the growth of similar sets of 

 organisms, so far as chemical functions are concerned, would be encouraged, and 

 that the products obtained thereby would be very similar in chemical characters. 



The actual quantity of carbon dioxide, and no doubt of nitric acid, but pro- 

 bably to a more limited extent, formed relatively to the quantity of oxygen 

 consumed during fermentation of similar volumes of different polluted waters, 

 would no doubt vary with the composition of the organic matters originally 

 present in them. But, for the purpose of water analysis, this would not affect the 

 value of the quantity of oxygen consumed, as a quantitative index of the amount 

 of the fermentable matters originally present, since this quantity would probably 

 exactly represent that which a similar volume of the same water would require 

 when discharged into a natural watercourse. 



It must be remembered, too, that, by the complete fermentation under aerobic 

 conditions of organic matters in an ordinary water, much the greater proportion 

 of the organic carbon and organic and ammoniacal nitrogen undergo complete 

 oxidation to carbon dioxide and nitric acid respectively, and only a very small 

 proportion of either of these constituents remains fixed as fermented organic 

 matters, and that variation in fermentative products due to selective chemical 



