108 Mr. John H. Balfour Broinie [March 17, 



water is liable to surface pollution, then filtration is a necessity, 

 and as in many other cases in this connection our ancestors were 

 wiser than they knew. Indeed, experience is often more valuable 

 than science, and is universally the foundation upon which all 

 safe science is built. Sand filters have been used in Britain 

 since about 1829. These filters are tanks from 6 to 8 feet deep. 

 Over the floor drain pipes or channels lead to the outlet pipe, and 

 over these we lay u layer of broken stones and gi-avel. These stones 

 are laid to a depth of 2 or 8 feet, the larger stones being at the 

 bottom of the tanks, and the smaller bearing the 2 feet of sand 

 which was supposed to l)e the filtering and purifying medium. The 

 water is allowed to percolate downwards at a certain slow rate, and 

 the effect is to remove mechanically certain matters in suspension. 

 For many years our chemical experts saw little or no value in sand 

 filtration, because in chemical analyses they found little or no dif- 

 ference between the filtered water and the unfiltered raw water. But 

 it was left to bacteriologists to find the real significance of sand 

 filtration. We know now that after use a jelly-like deposit is formed 

 on the top of the sand, and that that film has prevented the water 

 getting through the filters at the ordinary vertical rate of 4 to 6 

 inches an hour, or about 2h million gallons a day per acre of sand. 

 One ingenious engineer had the jelly scraped off the surface of his 

 filter, and the water flowed more freely certainly, but there were 

 within a few hours urgent telephonic messages from the bacterio- 

 logical department announcing the arrival of thousands of bacillus 

 coli, and that the water supplied by that company was not fit to 

 drink. Tlie fact is that it is the organic slime which is formed on 

 the top of the sand, and which the sand is only useful in supporting, 

 that is the effective agent in filtering the water, for the organic slime 

 destroys the micro-organisms which are in impure water. It is in 

 this respect that our ancestors in using their sand filters were wiser 

 than they knew. 



We know that Nature used to be thought red in tooth and claw, 

 and no doubt lions and tigers and ravening wolves appeal to our 

 trembling imaginations. But it is the still small animals that are the 

 real danger. We are taught by science to tremble at the sight of 

 the common house fly — t^^e missionary of typhoid and diarrhoea ; or 

 the flea from a certain rat which inoculates the plague. But it is the 

 micro-organisms, which are to be found in contaminated waters, and 

 which are so readily distributed by our system of town supply, that 

 are in fact the most dangerous foes of the human race. We have 

 seen, however, that Nature purifies our chalk waters, and also works 

 with, a will to save us pumping, but here again Nature by her 

 bacteriological laboratory toils for the health of the human race. 

 M. Duclaux has well said, " Whenever and wherever there is decom- 

 position of organic matter, whether it be the case of a weed or an 

 oak, of a worm or a whale, the work is exclusively performed by 



