104 SOURCE AND ASSIMILATION OF NITROGEN. 



it renders the soil porous, and, therefore, more per- 

 meable 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. 



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 

 volume, after having been completely dried under 

 the air-pump.* We have here an easy and satisfac- 

 tory 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 a means by which the necessary nitro- 

 gen is conveyed to plants. 



Nitrogen is found in lichens, which grow on basal- 

 tic rocks. Our fields produce more of it than we 

 have given them as manure, and it exists in all kinds 

 of soils and minerals which were never in contact 

 with organic substances. The nitrogen in these cases 

 could only have been extracted from the atmosphere. 



We find this nitrogen in the atmosphere in rain 

 water and in all kinds of soils, in the form of ammo- 

 nia, as a product of the decay and putrefaction of 

 preceding generations of animals and vegetables. 

 We find likewise that the proportion of azotized mat- 

 ters in plants is augmented by giving them a larger 

 supply of ammonia conveyed in the form of animal 

 manure. 



No conclusion can then have a better foundation 



* In experiments instituted by Dr. Daubeny, with a view of ascer- 

 taining whether vegetable mould had not the same property, he found 

 that both carbonic acid and ammoniacal gases were condensed within 

 its pores, as they would be by a lump of charcoal. 



