62 Dr. G. Reid. [May 7, 



12 minutes — the time occupied by the sewage in passing downward through 

 the first foot of filter. In my experience it is not unusual to find the free 

 ammonia figure reduced almost to an equal extent in effluents from fine- 

 grade filters, but, hitherto, I had no conception that the change was brought 

 about by so shallow a depth of filter. 



I may here mention that the rate of travel downwards of the sewage 

 through the filter was found, as the result of several observations, to vary 

 in accordance with the depth as follows : — 



From surface to 1 ft 12 mins. 



,. 1ft. „ 2 „ 12 „ 



« 2 „ ,, o „ 6 „ 



„ 3 „ „ 4 ft. 6 in 5 „ 



Total 35 „ 



This is a slow rate of travel compared with the rate in the case of large 

 particle filters, which, in my experience, in some cases allow the sewage 

 to pass through the entire depth of from 4 to 5 feet in about four minutes. 



The rate of travel was ascertained by a series of observations as follows : — 

 The delivery on the filter was stopped until the discharge pipes from the 

 respective trays showed unmistakable signs of diminished flow, when it was 

 started again, the period which elapsed before the discharge from the pipes 

 was restored being taken as the time occupied in the passage downward of 

 the sewage to the different depths. 



The free ammonia figure presents one other interesting feature. It will 

 be noticed that the progressive reduction which takes place during the 

 passage downwards through the first 3 feet is suddenly interrupted, a 

 considerable increase in the amount being recorded at a depth of 4 feet 

 6 inches. Accidental error in analysis does not explain this, for it will be 

 seen on referring to the detailed tables that this increase is invariably 

 recorded, and, as a matter of fact, I verified the accuracy of the results by 

 a second analysis in the case of the first few samples until it became quite 

 obvious that no such explanation of the occurrence was feasible. This 

 phenomenon will be referred to later in dealing with the general arguments 

 and conclusions. 



3. As regards the albuminoid ammonia figure, it will be seen that a 

 highly satisfactory reduction is effected by filtration through 1 foot only. 

 In fact, this figure and the oxygen absorbed and nitric nitrogen figures 

 considered together indicate a very high degree of purification, which 

 precludes the possibility of subsequent putrefactive change. 



4. It will be seen that the reduction in the oxygen absorbed is, stage by 



