Savage Precipitation. 57 



is about as complex a mixture as you can well have, but 

 not what I should call excessively concentrated, being as a 

 rule under 300 grains per gallon of solid matter. This 

 requires a little more of the precipitant than the Failsworth 

 sewage, but is easily coagulated and settles very quickly, 

 giving an effluent of very satisfactory clearness. A slight 

 filtration improves this sewage more than the former one, 

 but with some little time for settling it does not appear to 

 require filtration. 



The next is a very filthy sewage, from a cesspool in 

 Clayton, containing domestic and farm refuse, largely from 

 "piggeries." It requires about the same amount as the 

 Salford sample, and the precipitant throws down a com- 

 paratively enormous quantity of sediment, much of which 

 is brought out of solution. The supernatant liquor in this 

 sample is brighter-looking than the Salford one after 

 clarification, but its character is not so good. 



A notable point to be observed in the action of this 

 salt of iron is that it always brings a considerable amount 

 •of impurity out of solution, and, as this is in all cases of a 

 nitrogenous character, the albuminoid ammonia is very 

 considerably reduced, the purification in this respect being, 

 as a rule, from 80 to 90 per cent of alb. ammonia removed, 

 according to the general average of the samples I have 

 analysed. 



A striking illustration of this purification from nitro- 

 genous and, therefore, noxious decomposable matter is shown 

 by taking a sample of the water now in the Ship Canal 

 docks at Manchester. This has been settling for a long 

 time, and looks fairly clear, and is free from suspended 

 matter, and, in fact, does not appear to want any treatment. 

 The addition, however, of about 10 grains of the iron salt 

 to the gallon soon brings down a considerable sediment, 

 and the water thus purified is so clear that the original 

 appears of a very inferior character. 



