Vlii.] ABSORPTION OF NITROGENOUS COMPOUNDS 213 



but even pure sand will remove sodium chloride from 

 solution if the filtering column be sufficiently long; it 

 may, again, be illustrated by the phenomenon of 

 "laking," i.e., the power of certain colloid bodies, like 

 the hydrates of iron and alumina, on precipitation, to 

 drag down with themselves many organic substances 

 from solution. 



The absorption of the organic compounds of nitrogen 

 by the soil seems to be a physical process of this kind, 

 comparable to the action of charcoal in absorbing 

 ammonia or the strongly smelling products of putre- 

 faction, etc. The deodorising powers of earth for 

 faecal and decomposing matter are very familiar; this 

 means that fixation in a more or less insoluble and 

 non-volatile state of various organic nitrogen and sulphur 

 compounds is effected, and other inodorous nitrogen 

 compounds are retained in the same way. The 

 absorption is most marked with soils rich in humus 

 or in clay — the soil materials which present the 

 largest surface. The absorptive power of soil for 

 organic compounds of nitrogen is well seen in a 

 sewage farm, the object of which is to so far purify 

 sewage by percolation through a few feet of soil, as 

 to fit it to be turned into a river without danger to 

 health. For example, on the Manchester Sewage 

 Works, in 1900, percolation through 5 feet of soil 

 reduced the organic nitrogen in the liquid from 0-26 to 

 0-056 parts per million, and the free ammonia from 1-89 

 to 0-92. It is necessary, also, on a sewage farm to work 

 with soils possessing but a small absorbing power ; only 

 sandy and gravelly soils will permit of rapid enough 

 percolation, both to deal with large volumes of sew- 

 age and afterwards to aerate themselves and accom- 

 plish the destruction by bacterial action of the absorbed 

 material. Stiffer soils would be far more effectual 



