The Purification of Sewage by Bacteria. 33 



prepared for the plants before they can feed upon it. This all- 

 important work is performed by myriads of unpaid scavengers, 

 whose existence was not even suspected until late in the 

 seventeenth century. They are so tiny that it required the 

 powerful microscopes of the present day to make them, visible 

 at all ; they are known as " bacteria " or more familiarly as 

 " microbes." There are other low forms of life which bear a 

 part with them ; but it will simplify our task to-night if we 

 confine ourselves to bacteria and their work. It is they who 

 seize on the foul matter given off by the animal world, and 

 bring it into such a condition that it can support vegetable life. 

 This work is accomplished in at least two stages. The products 

 of animal life become first of all the prey of the decomposition 

 moulds and bacteria, whereby they are converted into the 

 various products of decomposition, such as ammonia and nitrites. 

 These, again, are seized upon by the other workers, which it is 

 convenient to classify as nitrifying bacteria, and converted by 

 them into nitrates, which I need hardly remind you, are among 

 the most valuable of our artificial fertilisers. 



As I shall point out later, the decomposition moulds and 

 bacteria are made use of in the septic tank to break down the 

 polluting matter of sewage ; and the work of purification is 

 completed by the nitrifying bacteria in the filters. 



When the sewage problem first began to make itself felt, the 

 eyes of sanitary authorities turned hopefully towards sewage 

 farms. They expected not only to get rid of their sewage, but 

 also to derive a profit from its utilisation on the land. The 

 creed of many sanitarians was summed up in the phrase '" The 

 rainfall to the river, the sewage to the land." These hopes 

 were, in nearly every instance, doomed to be blasted. In all 

 but a very small minority of instances, where local circumstances 

 were exceptionally favourable, the sewage farm has turned out 

 a source of \ ss instead of profit ; and in too many cases it has 

 completely failed to purify the sewage. Often enough it has 

 proved to be an unmitigated nuisance. The reason for this is 

 not far to seek. Land is undoubtedly the natural receptacle 

 3 



