90 SOURCE AND ASSIMILATION 



odour 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, litmus paper 

 held over them becomes blue. Soils, therefore, which 

 contain oxides of iron, and burned clay, must ab- 

 sorb ammonia, an action which is favoured by their 

 porous condition ; they further- prevent the escape 

 of the 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 ammonia absorbed by the clay or ferrugi- 

 nous 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 previously heated to 

 redness. Charcoal absorbs 90 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 72 times its 



