66 The Filtration Works for the Improvement of the 



explanation is certainly very ingenious, I leave the matter, 

 however, to be followed up by some one more competent than 

 myself to deal with it, and I pass from this part of the case with 

 the observation that the experience of some forty years shows 

 that the thicker layer of filtering material has given very satis- 

 factory results up to the present, and any saving to be effected 

 by using a smaller amount of material will be accompanied by 

 a loss of efficiency. 



Let me now come to another question respecting the effects of 

 filtration, and this is one of very great interest as well as im- 

 portance. I refer to the biological influence or effect produced 

 upon the filtered water by the process. Hitherto it has been 

 supposed that filtration through sand or, indeed, any other 

 material has not any effect in reducing the number of micro- 

 organisms present in the water. I have often seen it stated 

 that the pores of a sand filter are wide enough to allow 

 thousands abreast per square inch to pass down. A very 

 exhaustive series of experiments, made by Dr. Percy Frankland, 

 would appear to show that filtration through sand, coke, or 

 charcoal has, however, the effect of materially reducing the 

 number of micro-organisms in the water under treatment. The 

 original theory of filtration was that it cleared the water from 

 visible particles, but that it had no further beneficial effect. 

 Later investigations, however, showed that by the process the 

 organic matter in the water was reduced to an important 

 extent, and the inference was drawn that, therefore, the water 

 was made safer for drinking purposes. The danger was 

 regarded as existing in the organic matter, and medical men 

 argued that the latter, when large in quantity, in its passage 

 through the human system set up a state of intestinal disturb- 

 ance which often resulted in zymotic diseases of some of the 

 well-known forms. Thus, in the case of the Vartry water, 

 when first introduced into Dublin, doctors were said to hold it 

 responsible for a tendency to diarrhoea which prevailed for a 

 long time in the metropolis. Now, it is very hard to say 

 definitely whether organic matter in water is directly injurious 



