416 H. M. Vernon 



instead. For this purpose a larger superficial area of sand would 

 be required, and the rate of filtiation would bave to be slower. In 

 this case also a considerably greater depth of sand would be bene- 

 ficiai. Again, it wonld be necessary for the water, previous to its 

 passing through this filter, to pass through a smaller subsidiary one 

 of coarse sand, so as to remove the particles of suspended matter 

 in it. Thus this suspended matter, heiug mostly of a chlorophyll- 

 containing vegetable nature, would, wlien deposited on the sand in 

 the absence of light, rapidly undergo decomposition. The sand of 

 the subsidiary filter would therefore need to be frequently renewed, 

 hut that of the chief filter probably only seldom. 



With regard to the utilisation of sunlight as a purifying agent, 

 it would seem to be scarcely worth while to make special arrange- 

 ments to expose the water to its iufluence. No doubt some of the 

 bacteria present would be killed, but as we have seen, even several 

 hours' exposure does not effect very much in this way, whilst on 

 subsequently keeping the water in diffuse light, the unharmed germs 

 begin to multiply rapidly, so that the water becomes bacterially less 

 pure than betöre. 



In Order to fully utilise the purifying action of the bacteria in 

 the Avater, it would seem to be necessary to bave exceedingly large 

 reservoirs, in which the water could remain for days, and preferably 

 for weeks. Probably this would not be possible in most instances, 

 but in any case it would seem advisable to have the reservoirs as 

 large as possible. Great care should be taken to remove the Sediment 

 as frequently as possible, for, as an experiment previously described 

 proved, this deposit soon undergoes decomposition and effects a marked 

 contamination of the water. Supposing the plan were adopted of 

 filtering through sand the water which ran away from the tanks, 

 this source of impurity would of course be largely avoided. 



With regard to aeration, we have seen reason to think that 

 the purifying effect of this ageucy has been much over-rated. It 

 is of course necessary to keep up the percentage of oxygen in the 

 water, but a good deal would be done in this way if alg» could 

 be successfully maintained. There does not appear to be any very 

 simple method of determining the amount of oxygen dissolved in the 

 water; but it would certainly be worth while, should any doubt be feit 

 as to whether the aeration employed in a particular case were more 

 or less than was really necessary, to make daily determinations of 

 the oxygen for a period of some days or weeks, during which the 



