vm.] ABSORPTION OF NITROGENOUS COMPOUNDS 249 



characteristic property of colloids, and as we have 

 already seen (p. 39), the clay and humus of soil behave 

 as typical colloids. It has long been known that soils 

 and even pure sand will remove sodium chloride from 

 solution if the filtering column be sufficiently long ; but 

 the sand soon becomes saturated with salt and the 

 water will begin to come through more and more salty. 

 The soil materials have little adsorptive power for the 

 sodium or the chlorine ions. 



The absorption of the organic compounds of nitrogen 

 by the soil is purely one of adsorption, comparable to 

 the action of charcoal in absorbing ammonia or the 

 strongly smelling products of putrefaction, 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 colloid soil materials. 

 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 sewage and afterwards to aerate them- 

 selves and accomplish the destruction by bacterial 

 action of the absorbed material. Stiffer soils would 



