86 SOURCE AND ASSIMILATION 



ble of assimilation ; whilst the object of forest 

 culture is confined principally to the production of 

 carbon. All the various means of culture are sub- 

 servient to these two main purposes. A part only 

 of the carbonate of ammonia, which is conveyed by 

 rain to the soil is received by plants, because a 

 certain quantity of it is volatilised with the vapour 

 of water ; only that portion of it can be assimilated 

 which sinks deeply into the soil, or which is con- 

 veyed directly to the leaves by dew, or is absorbed 

 from the air along with the carbonic acid. 



Liquid animal excrements, such as the urine with 

 which the solid excrements are impregnated, con- 

 tain the greatest part of their ammonia in the state 

 of salts, in a form, therefore, in which it has com- 

 pletely lost its volatility when presented in this 

 condition ; not the smallest portion of the am- 

 monia is lost to the plants, it is all dissolved by 

 water, and imbibed by their roots. The evident 

 influence of gypsum upon the growth of grasses 

 the striking fertility and luxuriance of a meadow 

 upon which it is strewed depends only upon its 

 fixing in the soil the ammonia of the atmosphere, 

 which would otherwise be volatilised, with the 

 water which evaporates. The carbonate of ammo- 

 nia contained in rain-water is decomposed by 

 gypsum, in precisely the same manner as in the 

 manufacture of sal-ammoniac. Soluble sulphate 

 of ammonia and carbonate of lime are formed ; and 

 this salt of ammonia possessing no volatility is con- 



