ABSORPTION OF AMMONIA BY THE SOIL. 135 



the utility of which has long been known, although the cause of 

 its beneficial influence was not suspected. Gypsum powerfully 

 attracts ammonia from the atmosphere, and yields it again to 

 water which may soak through it ; so that as much ammonia, as 

 would supply the proportion of nitrogen to lOOlbs. of grass, is 

 yielded by little more than 41bs. of gypsum. The advantage of 

 manuring fields with burned clay, and the fertility of soils con- 

 taining iron, are to be referred to the same cause. Burned clay 

 has, like gypsum, the power of fixing ammonia from the atmo- 

 sphere, and of easily yielding it to water; and minerals containing 

 oxide of iron do the same, when separated into fine particles. 

 Powdered charcoal possesses a similar action, and, indeed, sur- 

 passes all other substances in its power of condensing ammonia 

 within its pores, absorbing 90 times its volume of this gas. 

 Decayed wood approaches very nearly to charcoal in this power ; 

 and vegetable mould, which principally consists of wood in a 

 more advanced state of decay, retains it in a very important 

 degree ; so that we perceive its influence on vegetation, to be by 

 no means confined to the supply of water and carbonic acid. 



197. On this account, vegetable mould is alone amply suffi- 

 cient for the cultivation of all Plants, which contain but an ave- 

 rage proportion of nitrogen ; but Corn can only be grown to the 

 greatest advantage, when the land is well manured with those 

 substances, which contain the largest proportion of ammonia. 

 This is yielded in the greatest abundance, by the excrements, 

 both fluid and solid, of Animals, and particularly of Man ; and 

 in China, where, from the immense population, it is necessary to 

 make the most of every foot of ground, the greatest care is taken 

 to preserve these ; and extraordinary fertility is the result. A 

 similar practice prevails on some parts of the Continent of 

 Europe ; and it is less successful, only because the mode of col- 

 lecting the materials allows of the escape of a large proportion 

 of the ammonia, before the manure is used. 



198. By a judicious system of management, large towns may 

 thus be rendered most important means of increasing the fertility 

 of a country, and therefore of contributing to the supply of whole- 

 some food ; instead of bringing together, as at present, so many 

 causes of misery and unhealthfulness. 





