II 



which was gradually burnt up by the oxygen liberated by 

 the green vegetation. 



Mr. Slater then draws the following conclusions from his 

 experiment. He says, the common belief seems to be 

 correct that green vegetation unaccompanied by white 

 fungoid growth is a favourable sign ; that sewage can 

 purify itself, but that the process is very slow ; that the speed 

 with which the impurities in a water are destroyed is greater 

 when it is spread out in shallow layers to the action of air 

 and light, and where its transparency is not interfered with 

 by suspended matter. He, however attaches little practical 

 importance to this property in sewage, the process being 

 extremely offensive, and such treatment in practice would be 

 ruinously costly. 



No doubt the purification would be more rapid if the 

 sewage was in motion in a river, and there found the water 

 plants developed and ready to oxidize the impurities. In 

 some cases the poisons from mining and manufacturing 

 towns would hinder the development of any vegetation 

 in rivers, into which these impurities were allowed to enter, 

 and in these this means of self-purification would be en- 

 tirely wanting, so far as it was produced by the plants they 

 contained. 



These experiments go to prove that all depends upon 

 the extent and the quality of pollution ; and where you 

 find a small river and a very large volume of sewage 

 pouring into it, it will have nearly destroyed all vegetation, 

 and one of the principal means of self-purification will only 

 be developed sparingly. But even if it is admitted that 

 self-purification is a fact, there seems to be now little doubt 

 that in the actual neighbourhood of the sewage intake, the 

 fish will in time succumb to an attack that, from our expe- 

 rience of the rapid increase of sewage, is daily becoming 



