ORIGIN AND ACTION OF HUMUS. 



excluded with the greatest care ? Can the 

 laws of life be investigated in an organised 

 being which is diseased or dying? 



The mere observation of a wood or mea- 

 dow is infinitely better adapted to decide so 

 simple a question than all the trivial experi- 

 ments under a glass globe ; the only dif- 

 ference is that instead of one plant there are 

 thousands. When we are acquainted with 

 the nature of a single cubic inch of their 

 scil, and know the composition of the air 

 and rainwater, we are in possession of all 

 the conditions necessary to their life. The 

 source of the different elements entering into 

 the composition of plants cannot possibly 

 escape us, if we know in what form they 

 take up their nourishment, and compare its 

 composition with that of the vegetable sub- 

 stances which compose their structure. 



All these questions will now be examined 

 and discussed. It has been already shown 

 that the carbon of plants is derived from the 

 atmosphere : it still remains for us to in- 

 quire what power is exerted on vegetation 

 by the numus of the soil and the inorganic 

 constituents of plants and also to trace the 

 sources of their nitrogen. 



CHAPTER III. 



ON THE ORIGIN AND ACTION OF HUMUS. 



IT will be shown in the second part of 

 this work, that all plants and vegetable 

 structures undergo two processes of decom- 

 position after death. One of these is named 

 fermentation; the other, putrefaction, decay, 

 or eremacausis* 



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 atmo- 

 sphere. 



The decay of woody fibre (the principal 

 constituent 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 re- 

 placed, its decay recommences, that is, it 

 again converts oxygen into carbonic acid. 

 Woody fibre consists of carbon and the ele- 

 ments of water ; and if we judge only from 

 the products formed during its decomposi- 

 tion, and from those formed by pure char- 

 coal, burned at a high temperature, we 

 might conclude that the causes were the 

 same in both : the decay of woody fibre pro- 

 ceeds, therefore, as if no hydrogen or oxy- 

 gen entered into its composition. 



A very long time is required for the com- 

 pletion of this process of combustion, and the 

 presence of water is necessary for its main- 

 tenance : alkalies promote it, but acids re- 

 tard it; all antiseptic substances, such as 

 sulphurous acid, the mercurial salts, empy- 

 reumatic oils, &c., cause its complete ces 

 sation. 



Woody fibre in a state of decay is U.e 

 substance called humus* 



The property of woody fibre t? ^ nvert 

 surrounding oxygen gas into carbotuc acid 

 diminishes in proportion as its decay ad- 

 vances, and at last a certain quantity of a 

 brown coaly-looking substance remains, in 

 which this property is entirely wanting 

 This substance is called mould; it is the 

 product of the complete decay of woody 

 fibre. Mould constitutes the principal of aL 

 the strata of brown coal and peat. 



Humus acts in the same manner in a soil 

 permeable to air as in the air itself; it is a 

 continued source of carbonic acid, which it 

 emits very slowly. An atmosphere of car- 

 bonic acid, formed at the expense of the air, 

 surrounds every particle of decaying humus. 

 The cultivation of land, by tilling and loos- 

 ening the soil, causes a free and unob- 

 structed access of air. An atmosphere of 

 carbonic acid is, therefore, contained in every 

 fertile soil, and is the first and most import- 

 ant food for the young plants which grow 

 in it. 



In spring, when those organs of plants 

 are absent which nature has appointed for 

 the assumption of nourishment from the 

 atmosphere, the component substance of the 

 seeds is exclusively employed in the forma- 

 tion of the roots. Each new radicle fibril 

 which a plant acquires may be regarded as 

 constituting at the same time a mouth, a 

 lung, and a stomach. The roots perform 

 the functions of the leaves from the first 

 moment of their formation : they extract 

 from the soil their proper nutriment, namely, 

 the carbonic acid generated by the humus. 



By loosening the soil which surrounds 

 young plants, we favour the access of air, 

 and the formation of carbonic acid; and, on 

 the other hand, the quantity of their food 

 is diminished by every difficulty which op- 

 poses the renewal of air. A plant itself 

 effects this change of air at a certain period 

 of its growth. The carbonic acid, which 

 protects the undecayed humus from farther 

 change, is absorbed and taken away by tbur 

 fine fibres of the roots, and by the root? 

 themselves ; this is replaced by atmospheric 

 air, by which process the decay is renewed, 

 and a fresh portion of carbonic acid formed. 

 A plant at this time receives its food both 

 by the roots and by theorgans aboveground, 

 and advances rapidly to maturity. 



When a plant is quite matured, and when 



. J 



* The word eremacausis was proposed by the 

 author some time since, in order to explain the 



true nature of decay; it is compounded from decomposition of humus by alkalies ; it does not 

 degrees and **vrii t burning. ' exist in the humus of vegetable physiologists. 



The humic acid of chemists is a product of the 



