OP THE NITROGEN OF PLANTS. 89 



explains why the action of the gypsum lasts for 

 several years. 



The advantage of manuring fields with burned 

 clay and the fertility of ferruginous soils, which 

 have been considered as facts so incomprehensible, 

 may be explained in an equally simple manner. 

 They have been ascribed to the great attraction for 

 water, exerted by dry clay and ferruginous earth ; 

 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 can- 

 not be serviceable either by the roots or leaves ? 

 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 pre- 

 cipitates 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 contain- 

 ing alumina or oxide of iron also possess, in an 

 eminent degree, the remarkable property of attract- 

 ing ammonia from the atmosphere and of retaining 

 it. Vauquelin, whilst engaged in the trial of a cri- 

 minal case, discovered that all rust of iron contains 

 a certain quantity of ammonia. Chevalier after- 

 wards 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. Bouis showed also, that the peculiar 



