OF THE CONSTITUENT ELEMENTS OF PLANTS. 25 



Hydrogen (^Ijiflammahle Air) is a very important 

 constituent of vegetable matter. It possesses a 

 special affinity for oxygen, with which it unites and 

 forms water. The whole of the phenomena of decay 

 depend upon the exercise of this affinity, and many 

 of the processes engaged in the nutrition of plants 

 originate in the attempt to gratify it. Hydrogen, 

 when in the state of a gas, is very combustible, and 

 the lightest body known ; but it is never found in 

 nature in an isolated condition. Water is the most 

 common combination in which it is presented ; and 

 it may be removed by various processes from the 

 oxygen, with which it is united in this body. 



Nitrogen * is quite opposed in its chemical char- 

 acters to the two bodies now described. Its princi- 

 pal characteristic is an indifference to all other sub- 

 stances, and an apparent reluctance to enter into 

 combination with them. When forced by peculiar 

 circumstances to do so, it seems to remain in the 

 combination by a vis inertice ; and very slight forces 

 effect the disunion of these feeble compounds. 



Yet nitrogen is an invariable constituent of plants, 

 and during their life is subject to the control of the 

 vital powers. But when the mysterious principle of 



* This gas was discovered in 1772, and is called also azote or azotic 

 gas J from the Greek, expressive of its being incapable of supporting 

 life. The name Nitrogen was given to it Som its entering into the 

 composition of nitric acid (aqua fortis). It has been suspected to be a 

 compound, but this has not been verified. The atmosphere is compos- 

 ed of four fiflhs nitrogen and one fifth oxygen, not, however, chemical- 

 ly united ; it also contains a ten thousandtn part of carbonic acid and 

 watery vapor. A mixture of oxygen and nitrogen in the proportions 

 named, exhibits the general properties of the atmosphere. Nitrogen 

 may be obtained from common air by removing its oxygen, and from 

 the lean part of flesh meat by boiling it in diluted nitric acid. It unites 

 with different proportions of oxygen, and forms as many distinct com- 

 pounds, viz. 



For other detjiils, see Webster's Chemistry, 3d edit., p. 134, &c; 



3 



