86 LECTURE V. 



Absorption of Ammonia. 



The air, as we have seen, occasionally contains minute 

 quantities of ammonia and nitric acid. Of these gases, the 

 former is given off during the combustion and decomposition 

 of nitrogenous organic substances, the latter is formed probably 

 by the direct combination of the nitrogen and oxygen of the 

 air which is effected by lightning-flashes. These gases do not, 

 however, exist isolated in the atmosphere : the ammonia will 

 combine with the nitric acid, and, in the absence of this acid, 

 it will combine with the carbonic acid of the air. 



The quantities of these substances present at any time in 

 the air are very small, and, inasmuch as they are very soluble 

 in water, and will therefore be washed out of the air by rain, 

 it is obvious that this source of combined nitrogen cannot be 

 of any great importance to the plant. Still the question as to 

 whether or not plants can absorb ammonia in the gaseous form 

 from the air is an interesting one, and one which has received 

 the attention of many observers. The researches of Ville 

 prove that ammonia can be absorbed from the air by the leaves, 

 but that this absorption is very small under ordinary circum- 

 stances is shewn by an experiment of Boussingault. A Dwarf 

 Runner was planted in a soil containing no nitrogen and left 

 to grow in the open air, protected however, from the rain, for 

 nearly four months ; it was then estimated that the plant had 

 gained 0*003 l g rm - of nitrogen during the experiment. Sachs 

 found that a plant absorbed ammonium carbonate (probably 

 carbamate) when it was present in the air, and this result has 

 been since confirmed by Mayer and by Schlosing. 



The following are some of Mayer's results. In his experiments, 

 Cabbage-plants of approximately the same weight were grown under 

 various conditions, and the proportion of nitrogen present in each was 

 determined at the close of the experiment : one of the plants was analysed 

 at the beginning of the experiment, to afford a means of comparison. 



Dry weight N. ^/N^ 



Plant at commencement of expt. 0-364 grm. croioo 27 



grown in open air end of expt. 0713 0*0128 r8 



(0715 0*0138 i'Q 



receiver without NH 3 \ 



(0779 0-0129 17 



fl'OQO ,. 0-0240 2 '2 



with NH 3 



[1-562 0-0380 2-4 



