ORGANIC CONSTITUENTS OF PLANTS. 109 



213. Oxygen (0) is another element of plants. Air contains about 

 21 per cent, of it. Every 9 Ibs. of water contain 8 of oxygen, and it 

 is combined with various elements, so as to form a great part of the 

 solid rocks of the globe, as well as of the bodies of animals and man. 



214. It is chiefly in its state of combination with Hydrogen (H), so 

 us to form water (HO), that oxygen is taken up by plants. Hydrogen 

 is not found in a free state in nature, and with the exception of coal, 

 it does not enter into the composition of the mineral masses of the 

 globe. It forms ^ of the weight of water, and it is present in the 

 atmosphere in combination with nitrogen. Hydrogen is also furnished 

 by sulphuretted hydrogen, and some compounds of carbon. 



215. Nitrogen (N) is another element found in plants. It forms 79 

 per cent, of the atmosphere, and abounds in animal tissues. The latter, 

 during their decay, give off nitrogen, combined with hydrogen, in the 

 form of ammonia (NH 3 ), which is absorbed in large quantities by 

 carbon, is very soluble in water, and seems to be the chief source 

 whence plants derive nitrogen. In tropical countries where thunder 

 storms are frequent, the nitrogen and oxygen of the air are sometimes 

 made to combine, so as to produce nitric acid, (NO 5 ) which, either in 

 this state, or in combination with alkaline matters, furnishes a supply 

 of nitrogen. Daubeny thinks that the ammonia and carbonic acid in 

 the atmosphere are derived in part from volcanic actions going on in the 

 interior of the globe. The continued fertility of the Terra del Lavoro, 

 and other parts of Italy, is attributed by him to the disengagement of 

 ammoniacal salts and carbonic acid by volcanic processes going on un- 

 derneath ; and to the same source he traces the abundance of gluten 

 in the crops, as evidenced by the excellence of Italian macaroni. 



216. Mulder maintains that the ammonia is not carried down from 

 the atmosphere, but is produced in the soil by the combination between 

 the nitrogen of the air, and the hydrogen of decomposing matters. The 

 same thing takes place, as in natural saltpetre caverns of Ceylon, with 

 this exception, that, by the subsequent action of oxygen, ulmic, humic, 

 geic, apocrenic, and crenic acids, are formed in place of nitric acid. 

 These acids consist of carbon, oxygen, and hydrogen, in different pro- 

 portions, and they form soluble salts with ammonia. By all porotis 

 substances like the soil, ammonia is produced, provided they are 

 moist, and filled with atmospheric air, and are exposed to a certain 

 temperature. It is thus, he states, that moist charcoal and hunms 

 become impregnated with ammonia. 



217. These four elementary bodies then are supplied to plants, chiefly 

 in the form of carbonic acid (CO 2 ), water (HO), and ammonia (NH 3 ). 

 In these states of combination they exist in the atmosphere, and hence 

 some plants can live suspended in the air, without any attachment to 

 the soil. When a volcanic or a coral island appears above the waters 

 of the ocean, the lichens which are developed on it are nourished 



