USE OF BURNED CLAY AS A MANURE. 103 



in as great a degree : and bedsides, what influence 

 can be ascribed to a hundred pounds of water spread 

 over an acre of land, in a condition in which it can- 

 not be serviceable either by the roots or leaves 1 

 The true cause is this : — 



The oxides of iron and alumina are distinguished 

 from all other metallic oxides by their power of form- 

 ing solid compounds with ammonia. The precipi- 

 tates obtained by the addition of ammonia to salts 

 of alumina or iron are true salts, in which the ammo- 

 nia is contained as a base. Minerals containing alu- 

 mina or oxide of iron also possess, in an eminent de- 

 gree, the remarkable property of attracting ammonia 

 from the atmosphere and of retaining it. Vauquelin, 

 whilst engaged in the trial of a criminal case, discov- 

 ered that all rust of iron contains a certain quantity of 

 ammonia. Chevalier afterwards found that ammonia 

 is a constituent of all minerals containing iron ; that 

 even hematite, a mineral which is not at all porous, 

 contains one per cent, of it. Bonis 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 ammo- 

 nia, 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 fa- 

 vored by their porous condition ; they further pre- 

 vent the escape of the ammonia once absorbed, by 

 their chemical properties. Such 'soils, in fact, act 

 precisely as a mineral acid woufd do, if extensively 

 spread over their surface; with this difference, that 

 the acid would penetrate the ground, enter into com- 

 bination 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 influence; 



