Adeney — Dissolved Gases and Fermentative Changes. 617 



In reference to this point it must be remembered that during efficient filtration 

 all but traces of peaty or humus matters are separated. Hence, the absence of all 

 but traces of these matters and of ammonia may be accepted as one proof of 

 efficient filtration in water. 



That efficient filtration of all water to be used for dietetic purposes is a prudent 

 precaution in the interests of health must be accepted from the property which 

 bacteriologists have shown properly constructed filters to possess, of separating 

 pathogenic organisms from water (Koch on Cholera, 1894). 



It will be observed that the above two conditions in first-class potable water 

 may be ascertained by the methods in ordinary use for water analysis. 



In cases of doubt or suspicion, however, a careful examination of the character 

 of the organic matters present should be made. 



There can, I think, be no question that the method I have employed is capable 

 of revealing both the nature and amount of polluting matters, should they be 

 present in a water, or of proving their absence if not present. I can state from 

 my own experience that examinations of such suspicious waters have afforded me 

 results which have left no doubt in my mind as to their suitability or otherwise 

 for dietetic purposes. 



When we turn to the consideration of polluted waters, the matter assumes an 

 entirely different aspect, for it is quite evident from the experiments which I have 

 described, that the methods of analysis of potable waters, when applied to that of 

 polluted waters, are not only quite incapable of indicating the true character of 

 the polluting matters, but that they may even lead to absolutely misleading results. 



From the experiments with polluted waters which I have recorded, it may be 

 gathered that it is essential, in order to predict the action which a given volume 

 of a particular polluted water will exert, when discharged into a watercourse of 

 known volume and flow, to determine beforehand the exact volume of atmospheric 

 oxygen which will be required for the comjjlete fermentation of the polluting 

 matters present, and that it is necessary also for technical purposes to approxi- 

 mately determine what fraction of that volume of oxygen is required for the 

 first stage of fermentation of these matters. Under no circumstances ought 

 a polluted water be discharged into a watercourse in such volumes that the 

 dissolved oxygen in the mixture of the two waters, pure and polluted, is not 

 more than sufficient to supply that required for the first-stage fermentation of 

 the polluting matters. 



This I consider is necessary, because the first stage of fermentation of easily 

 fermentable organic matters proceeds much more rapidly in a water exposed to 

 the atmosphere than does the solution and diffusion of atmospheric oxygen 

 through the water. Hence, if the quantity of dissolved oxygen of a water is 

 insufficient for the needs of this stage of fermentation of any polluting matters in 

 it, putrefactive fermentation will set in. ......... 



