64 ON THE ORIGIN AND ACTION OF HUMUS. 



It will likewise be shown, that decay is a slow 

 process of combustion, — a process, therefore, in 

 which the combustible parts of a plant unite with 

 the oxygen of the atmosphere. 



The decay of woody fibre (the principal constit- 

 uent of all plants) is accompanied by a phenomenon 

 of a peculiar kind. This substance, in contact with 

 air or oxygen gas, converts the latter into an equal 

 volume of carbonic acid, and its decay ceases upon 

 the disappearance of the oxygen. If the carbonic 

 acid is removed, and oxygen replaced, its decay 

 recommences, that is, it again converts oxygen into 

 carbonic acid. Woody fibre consists of carbon and 

 the elements of water ; and if we judge only from 

 the products formed during its decomposition, and 

 from those formed by pure charcoal, burned at a high 

 temperature, we might conclude that the causes 

 were the same in both: the decay of woody fibre 

 proceeds, therefore, as if no hydrogen or oxygen 

 entered into its composition.* 



A very long time is required for the completion 

 of this process of combustion, and the presence of 

 water is necessary for its maintenance ; alkalies 

 promote it, but acids retard it; all antiseptic sub- 

 elements of a body with the oxygen of the air ; a slow combustion or 

 oxidation. 



The conversion of wood into humus, the formation of acetic acid 

 out of alcohol, nitrification, and numerous other processes, are of this 

 nature. Vegetable juices of every kind, parts of animal and vegetable 

 substances, moist sawdust, blood, &c., cannot be exposed to the air, 

 without suffering immediately a progressive change of color and prop- 

 erties, during which oxygen is absorbed. These changes do not take 

 place when water is excluded, or when the substances are exposed to 

 the temperature of 32°, and different bodies require different degrees 

 of heat, in order to effect the absorption of oxygen, and, consequently, 

 their eremacausis. The property of suffering this change is possessed 

 in the highest degree by substances which contain nitrogen. — Liebig. 

 Org. Chem. Part 2d. 



* In the Appendix to the Third Report of the Agriculture of Massa- 

 chusetts, 1840, Dr. S. L. Dana adduces the following example, to show 

 that even a moist plant will not decay, if air is excluded. A piece of 

 a white birch tree was taken from a depth of twenty-five feet below 

 the surface, in Low^ell. **It must have been inhumed there probably 

 before the creation of man, yet this most perishable of all wood is 

 nearly as sound as if cut from the forest last fall." 



