BURNING LAND. 19 



manner. They have been ascribed to the great attraction for 

 water exerted by dry clay and ferruginous earth j but common 

 dry, arable land possesses this property in as great a degree ; and, 

 besides, what influence can be ascribed to a hundred pounds of 

 water, spread over an acre of land, in a condition in which it 

 cannot be serviceable either by the roots or leaves ? 



&quot; The true cause is this : The oxides of iron and alumina 

 are distinguished from all other metallic oxides by their power 

 of forming solid compounds with ammonia. The precipitates 

 obtained by the addition of ammonia to salts of alumina or iron 

 are true salts, in which the ammonia is contained as a base. 

 Minerals containing alumina, or oxide of iron, also possess in an 

 eminent degree the remarkable property of attracting ammonia from 

 the atmosphere, and retaining it. Vauquelin discovered that all 

 rust of iron contains a certain quantity of ammonia. Chevalier 

 found that ammonia is a constituent of all minerals containing 

 iron ; and that even hematite, which is not at all porous, contains 

 one per cent, of it. Bouis showed, also, that the peculiar odor 

 observed on moistening minerals containing alumina is partly 

 owing to their exhaling ammonia. Indeed gypsum, and some 

 varieties of alumina, pipe-clay for example, emit so much 

 ammonia, when moistened with caustic potash, that, even after 

 they have been exposed for two days, reddened litmus paper held 

 over them becomes blue. Soils, therefore, which contain oxides 

 of iron and burned clay must absorb ammonia an action which 

 is favored by their porous condition. They further prevent the 

 escape of ammonia, once absorbed by their chemical properties. 

 Such soils, in fact, act precisely as a mineral acid would do, if 

 extensively spread over their surface ; with this difference, that 

 the acid would penetrate the ground, enter into combination 

 with lime, alumina, and other bases, and thus lose in a few 

 hours its property of absorbing ammonia from the atmosphere. 

 The addition of burned clay to soils has also a secondary in 

 fluence. It renders the soil porous, and therefore more perme 

 able to air and moisture. The ammonia absorbed by the clay 

 or ferruginous oxides is separated by every shower of rain, and 

 conveyed in solution to the soil.&quot;* 



I have gone thus at large into the subject of paring and burn- 



* Liebig, Agricultural Chemistry, Boston ed. p. 102. 



