56 SOURCE AND ASSIMILATION OF NITROGEN. 



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 of retaining it. 

 Vauquelin, whilst engaged in the trial of a criminal case, dis- 

 covered that all rust of iron contains a certain quantity of 

 ammonia. Chevalier afterwards found that ammonia is a con- 

 stituent 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 odor observed on moistening 

 minerals containing alumina, is partly owing to their exhaling 

 ammonia. Indeed, many kinds of gypsum and some varieties 

 of alumina, pipe-clay for example, emit so much ammonia, when 

 moistened with caustic potash, even after they have been exposed 

 for two days, that reddened litmus paper held over them becomes 

 blue. Soils, therefore, containing oxides of iron, and burned 

 clay, must absorb ammonia, an action which is favored by their 

 porous condition ; they further prevent, by their chemical pro- 

 perties, the escape of the ammonia once absorbed. Such soils, 

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

 spread over their surface. 



The ammonia absorbed by the clay of ferruginous oxides is 

 separated by every shower of rain, and conveyed in solution to 

 the soil. 



Powdered charcoal possesses a similar action, but surpasses all 

 other substances in the power which it possesses of condensing 

 ammonia within its pores, particularly when it has been previ- 

 ously heated to redness. Charcoal absorbs ninety times its 

 volume of ammoniacal gas, which may be again separated by 

 simply moistening it with water. (De Saussure.) Decayed 

 wood approaches very nearly to charcoal in this power ; decayed 

 oak wood absorbs seventy-two times its volume of this gas, after 

 having been completely dried under the air-pump. We have 

 here an easy and satisfactory means of explaining still further 

 the properties of humus, or wood in a decaying state. It is not 

 only a slow and constant source of carbonic acid, but it is also 



