THE VOLATILE PART OF PLANTS. 87 



matters at common temperatures is strictly analogous in a 

 cheirffbal sense to actual burning, Liebig has proposed the 

 term eremacausis, (slow burning), to designate the chemi- 

 cal process which takes place in decay and putrefaction, 

 and which is concerned in many transformations, as in the 

 making of vinegar and the formation of saltpeter. 



Oxygen is necessary to organic life. The act of breath- 

 ing introduces it into the lungs and blood of animals, 

 where it aids the important office of respiration. Ani- 

 mals, and plants as well, speedily perish if deprived of 

 ,free oxygen, which has therefore been called vital air. 



Oxygen has a universal tendency to combine with other 

 substances, and form with them new compounds. With 

 carbon, as we have seen, it forms carbonic acid. With 

 iron, it unites in various proportions, giving origin to sev- 

 eral distinct oxides, of which iron-rust is one, and anvil- 

 scales another. In decay, putrefaction, fermentation, and 

 respiration, numberless new products are formed, the re- 

 sults of its chemical affinities. 



Oxygen is estimated to be the most abundant body in 

 nature. In the free state, but mixed with other gases, it 

 constitutes one-fifth of the bulk of the atmosphere. In 

 chemical union with other bodies, it forms eight-ninths of 

 the weight of all the water of the globe, and one-third of 

 its solid crust its soils and rocks, as well as of all the 

 plants and animals which exist upon it. In fact there are 

 but few compound substances occurring in ordinary expe- 

 rience into which oxygen does not enter as a necessary 

 ingredient. 



Nitrogen. This body is the other chief constituent of 

 the atmosphere, in which its office might appear to be 

 mainly that of diluting and tempering the affinities of 

 oxygen. Indirectly, however, it serves other most impor- 

 tant uses, as will presently be seen. 



For the preparation of nitrogen we have only to remove 

 the oxygen from a portion of atmospheric air. This may 



