BY THE HON. W. F. TAYLOR, M.D., ETC. 39' 



fine jelly, which is capable of intercepting the very smallest, 

 bacteria, if there is no rupture or loss of continuity in the 

 material. This gelatinous material which forms on the top 

 layer of sand, consists, no doubt, very largely of intercepted 

 organic matter and saprophytic bacteria. It becomes sufficiently 

 well formed in two or three days after removing the top layer of 

 sand, to become effective in arresting bacteria. For the first, 

 two or three days, therefore, after removing the sand, the filter- 

 bed is not effectual in intercepting bacteria. There is no evi- 

 dence of filter-beds becoming reduced in efficiency by prolonged 

 use, even for a period of 68 days. It would seem that the 

 organisms tend to grow deeper and deeper into the beds, and 

 might possibly in time grow quite through the filter and 

 re-appear in the filtered water — but owing to the thickness of 

 the filteruig medium this must necessarily occupy a long time. 

 The reason why the top layers of sand should be skimmed and 

 removed periodically is to prevent the filter-beds being overtaxed, 

 because filtration becomes slow in old beds owing to the thick- 

 ness of the gelatinous coating and clogging of the top layers. 

 This gelatinous colloidal mass of bacteria with which the filter- 

 bed becomes covered has the power of arresting 97*5 per cent. 

 or more of the bacteria in the filtering water. 



The process of filtration is chemical as well as mechanical 

 and bacteriological. The late Colonel Sir Francis Bolton, in 

 his instructive work on the London Water Supply, says : — " It 

 is well known that all solid bodies attract about them an atmos- 

 pheric film, and therefore as a bed of sand and gravel is an 

 agglomeration of minute stones, each with its coating of com- 

 pressed air (or in other words, compressed oxygen and nitrogen), 

 the water filtering through its interstices has to pass a concen- 

 trated body of oxygen capable of dei!omposing it and forming 

 other compounds ; consequently, if we take the case of a decayed 

 leaf for example, we can see that ii could be resolved to some 

 extent into carbon, nitrogen, and hydrogen, which re-combining 

 with the oxygen, form carbonic acid gas, ammonia and water. 

 As the result of this chemical process the polluting vegetable 

 matter will have actually vanished, and though the filter-bed 

 has really abstracted it from the water the bed itself will show 

 no trace of it." 



