488 Toion Sewage. 



lime, indeed, there was more in a gallon of drainage than of 

 sewage : the like may be said of sulphuric acid. Lastly, of 

 soluble silica a notable portion passed off in the drainage. 



Of organic matter in solution a very considerable quantity was 

 found in the drainage-water, but of so different a character as to 

 suggest the probability that it was derived from vegetable matter 

 within the soil, rather than directly from the sewage, especially 

 in periods of active vegetation. 



It is very important to remark that the drainage contained 

 more nitrogen in the form of nitric acid than as ammonia, whereas 

 the sewage scarcely contained an apprecialjle amount in that form ; 

 that is to say, the soil had retained less nitrogen than would have 

 been supposed if only the more partial analyses had been made. 



It appears, then, that the constituents of the most value had 

 been most efficiently retained ; but nevertheless the sewage had 

 neither been perfectly deprived of its manurial elements nor 

 perfectly purified. 



There is, indeed, a limit to the power which a soil possesses 

 of removing substances from solution, or of preventing those 

 already absorbed from being dissolved in water passing through 

 it, the result being dependent on the- physical and chemical 

 character of the soil itself, and on the amount and composition of 

 the fluid passing through it. 



Where the land is covered with luxuriant vegetation there 

 will probably always be a certain amount of soluble organic 

 matter derived from that source in the drainage-water. 



So far, however, as the nitrogen in the drainage exists in the 

 form of nitric acid, it is a pretty satisfactory indication that the 

 organic matter has to a great extent already passed the stage of 

 deleterious putrescence. 



The arrangements at Rugby did not allow of the water drained 

 from one portion of the land being passed over another ; but 

 at Beddington, near Croydon, a great portion of the water does 

 duty twice and sometimes three times, and it consequently passes 

 from the land in a state of much greater purity than the Rugby 

 drainage-water, as the following Table, framed from results com- 

 municated by Mr. Latham (engineer to the Croydon Board of 

 Health), will show. (See following page.) 



About the same amount of ammonia was found on the average 

 in the seioage of Croydon as in that of Rugby ; but in the 

 Croydon drainage the amount was extremely small. It is unfor- 

 tunate that the quantity of nitric acid was not also determined ; 

 but we are informed that it undoubtedly exists in some amount 

 in the drainage from the Beddington meadows. Still, although 

 formerly the Croydon Board had to meet numerous law-suits on 

 account of the pollution of the river by the sewage, the fluid is 



