308 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



(2) That after being filtered tlirough filter paper it should 

 not absorb more than : — 



(a) 0-5 part by weight per 100,000 of dissolved or atmo- 

 spheric oxygen in twenty-four hours ; 



(b) I'O part by weight per 100,000 of dissolved or atmo- 

 spheric oxygen in forty-eight hours ; or, 



(c) r5 parts by weight per 100,000 of dissolved or atmo- 

 spheric oxygen in 5 days/ 



Although these tests are open to some criticism in matters 

 of detail, they do broadly serve to determine whether an 

 effluent is likely to give rise to nuisance or not. They may be 

 hardly stringent enough for special cases, e.g., if the effluent 

 enters a stream used for water supply ; or on the other hand 

 may be unnecessarily severe, when ample dilution takes place, 

 and the water into which the effluent is discharged is not used 

 for drinldng. 



The adequacy or otherwise of the purification effected 

 under given conditions can generally be judged from a careful 

 examination of the conditions obtaining at the point of dis- 

 charge, especially the various forms of living growth which can 

 be there observed. The presence of Beggiatoa, for instance, 

 would indicate that unoxidised sulphides are still present, 

 and, consequently, that the purification was imperfect. Such 

 a state of things is almost certain to give rise to nuisance. 

 Other forms of sewage fungus, such as Sphaerotilus natans or 

 Leftomitus lacteus, are also characteristic of imperfect purifica- 

 tion. Certain protozoa, such as carchesium, indicate a more 

 satisfactory purification, still stopping short, however, of 

 complete mineralisation. A first-class effluent can generally 

 be recognised by an increased development of healthy aquatic 

 vegetation in its vicinity, owing, doubtless, to the nitrates 

 present. 



The Analysis of Water.— Turning now to the subject of 

 the analysis of water, while it is true that water may contain 



