THE VOLATILE PAKT OF PLANTS. 21 



in a chemical sense to actual burning, Liebig has pro- 

 posed the term eremacausis (slow burning), to designate 

 the chemical process of oxidation which takes place in 

 decay, and which is concerned in many transformations, 

 as in the making of vinegar and the formation of salt- 

 peter. * 



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 nearly universal tendency to combine 

 with other substances, and form with them new com- 

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

 acid gas or carbon dioxide. With iron it unites in vari- 

 ous proportions, giving origin to several distinct oxides. 

 In decay, putrefaction, fermentation, and respiration, 

 numberless new products are formed, the results 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 or- 

 dinary experience into which oxygen does not enter as a 

 necessary ingredient. 



Nitrogen. This body is the other chief constituent of 

 the atmosphere, of which it makes up about four-fifths 

 the bulk, and in which its office would appear to be 



* Recent investigation has demonstrated that the oxidations which 

 Liebig classed under the term cremacausis. arc for the most part strict- 

 ly dependent on the vital processes of extremely minute organisms, 

 which arc in general characterized by the terms microbes or micro- 

 denies, and arc more specitically designated bacteria, i. e., "rod-shaped 

 animalcules or plaiitlets." 



