6o The Filtration Works for the Improvement of the 



receives its burden of earthy impurities, and also those of a 

 vegetable character — due to decaying leaves, weeds, grass, and 

 perhaps a little bog stuff. Happily in the case of catchment 

 area water there is not much danger from animal pollution, 

 because the sources of such contamination are generally too few 

 to have any material effect on the quality of the resultant 

 water. Now, when the raw material, so to speak, is of such 

 a kind, water authorities have in the past had rather rough 

 times of it with their consumers, and, as the outcome of full 

 investigation, sand filtration has been adopted in a large number 

 of towns. There are, of course, other ways known of freeing 

 water to a greater or less extent from the objectionable matters 

 contained in it. I have already spoken of subsidence. There 

 is also the aeration process, such as passing the water over a fall 

 or weir. And, again, a new process has been tried by which 

 an electric current is passed through the water, with the result 

 that the organic matter in suspension is carried down as a 

 precipitate. Most authorities, however, hold that filtration in 

 some form or other is the only practicable mode of removing 

 objectionable impurities from drinking water. And as my time 

 is limited, I must not detain you by saying anything upon other 

 methods of purification. To pass on, then, our first idea of 

 filtration was derived from nature. Thus we have, no doubt, 

 seen a well sunk near a river bank giving bright pleasant 

 water, whilst that in the river was too foul to drink, or even to 

 wade in. Again, one part of a river may be in a clay district, 

 and the water muddy and unusable ; but further down, with a 

 slower velocity, and the water passing through gravel and sand, 

 we get the water as clear as could be desired. After a good deal 

 of failure, and a number of very costly experiments, the modern 

 sand filter used by water authorities has resulted ; and I think 

 I may say with perfect accuracy, that when properly made and 

 carefully worked it is a very efficient means of improving the 

 quality of a town supply. The present system was not, however, 

 settled all at once. We had lateral filtration, upward filtration, 

 and finally downward filtration, and the latter system has taken 

 the foremost place. 



