244 The Poioer of Soils to 



would greatly tend to accelerate. Hence it is not to be ex- 

 pected that any soil, however constituted, could, by mere 

 exposure to the air, ever become very rich in ammonia, or 

 indeed contain more than a minute trace of it ; because the 

 entire quantity which it could possibly receive in any one 

 day could only be very minute ; and though it is true that 

 the constant repetition of such additions to the soil would in 

 time amount to a considerable quantity ; yet, as it would 

 take a long time, and as, during the whole of this time, the 

 soil would be necessarily exposed to the influence of evapo- 

 ration, and the other causes, whatever they may be, which 

 tend to remove ammonia again from the soil ; so it is not to 

 be expected that, even under the most favorable circumstan- 

 ces, any large accumulation could take place. 



It will be an important subject of inquiry, to ascertain 

 how much ammonia a good clay soil is able to absorb under 

 the most favorable circumstances ; for if the quantity is suffi- 

 ciently large, it would unquestionably be worth while to 

 form large filter beds in the reservoir of drainage water from 

 towns ; not as has often been proposed, merely for the pur- 

 pose of purifying it from all the solid matter mechanically 

 suspended in it, but also to arrest the ammonia and other 

 valuable soluble elements of manure which it contains. Some 

 of Mr. Way's experiments on this subject are very curious. 

 He not only found that by filtering a portion of putrid drain- 

 age water through a few inches of soil, it had lost all bad 

 odor, and contained no longer any ammonia ; but he also 

 found that on filtering fresh liquids of this sort, which had 

 not yet begun to putrefy, through such a layer of soil, they 

 even lost all tendency to putrefy, and might then be kept 

 weeks exposed to the air without their showing any indica- 

 tion of putrefaction. Although the chief agent in producing 

 these remarkable effects is clay, yet it is evident that such 

 filtering beds, if worth making at all, could not be made of 

 clay alone, because, from its close tenacious nature, and the 

 extreme slowness with which it allows the percolation of 

 water, it would be evidently unfit for any kind of filter ; and 

 a mixture of clay and sand, such as we find in a good mod- 



