VEGETATION. 345 



other element, but is free ; and as it bathes the leaves it 

 would seem that it might be absorbed as the carbonic an- 

 hydride is. But not a particle of the nitrogen, so far as we 

 know, is absorbed by them. How, then, the question re- 

 turns, does the plant get its nitrogen ? It comes from the 

 soil. But how ? There is no free nitrogen in the soil, so 

 that the mouths of the roots may drink it up as they do 

 carbonic anhydride. But there are substances in the soil 

 which contain nitrogen in combination, and furnish it to 

 the plant. The principal of these is ammonia, which, as 

 you learned in 160, is composed of nitrogen and hydro- 

 gen. In the process of decay always going on in the soil 

 there are produced ammonia, by the union of nitrogen and 

 hydrogen, and carbonic anhydride by the union of carbon 

 and oxygen. Then the ammonia and carbonic anhydride 

 unite with the elements of water to form carbonate of am- 

 monium. As this salt is volatile, much of it escapes into 

 the air ; but it is brought down to the earth again by the 

 dew and the rain. The existence of ammonia in rain-water 

 has been proved by Liebig. The amount is indeed very 

 small in any one quantity of water subjected to examina- 

 tion ; but the aggregate for the year is so large an amount 

 that we may say that the land receives great quantities 

 of one of its most valuable fertilizers from the rains of 

 heaven. This is but returning, however, to the ground 

 what is first generated there by decay. The value of ma- 

 nures containing ammonia will be spoken of hereafter. 



478. Summary. You see, then, that from carbonic anhy- 

 dride, water, and ammonia all the constituents of plants 

 can be furnished, for we have in these all the elements 

 which compose these constituents. We may state it thus : 



Carbonic anhydride gives carbon, oxygen, )- . . , (wood, starch, 

 Water hydrogen, oxygen, I JJ23 \ gum, gluten, 



Ammonia " nitrogen, hydrogen, ) ai >a ( sugar, etc. 



P2 



