THE VOLATILE PART OF PLANTS. 387 
matters at common temperatures is strictly analogous in a 
chemical 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 neccessary. 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 
