On the Jpjjllcation of Manures. 



251 



dition in which neither alkalies nor earths can convert it into 

 humic acid, nor render it soluble. 



Now humus, in common with all carbonaceous substances 

 which contain a small proportion of hydrogen, undergoes, when 

 slightly moistened, a process of slow combustion, which may con- 

 sist, either simply in an absorption of oxygen, in which case no- 

 thing will be given off, or in the production of carbonic acid, when 

 of course that gas will be disengaged. 



The slowness with which the latter takes place, and the absence 

 of that sensible heat and light which accompany ordinary com- 

 bustion, have led Professor Liebig to distinguish this process 

 by the term erema-causis, from Yipe/xa by degrees, and xauai^ 

 burning. 



Now this slow combustion goes on in w^oody fibre, from the 

 first commencement of its decay till it passes into that description 

 of humus before described, which is insoluble in earths and 

 alkalies. 



During the intermediate stages it constitutes humus of the 

 former description, which, as already stated, when acted upon by 

 these substances, is converted into humic acid, and combines 

 with them. 



During the whole period of its decay, until it has reached that 

 ultimate point at which it ceases to be soluble, and has become a 

 Idnd of caput mortuum, it goes on continually disengaging car- 

 bonic acid ; so that the roots of plants fixed in humus of this 

 quality are surrounded by an atmosphere of the gas in question, 

 which is therefore held in solution by the water taken up by them 

 as sap. 



Humus, then, by means of the carbonic acid which it emits, 

 does supply food indirectly to the plant; but that it communi- 

 cates nothing to it directly appears from the fact that it is itself 

 almost wholly insoluble in water, and can only be dissolved by it 

 when in union with an alkali or an earth. Hence the amount of 

 humus, or humic acid, which enters the vegetable tissue, must at 

 least be limited by the quantity of earthy and alkaline matters 

 which are absorbed ; and how small that cjuantity is may be seen 

 by reference to the statement I have already made, as to the 

 weight of ashes in a given portion of a plant. 



We seem, therefore, to be justified in attributing the carbon 

 which plants contain exclusively to the decomposition of carbonic 

 acid, which, before the leaves are developed, is introduced toge- 

 ther with the sap by the roots, but afterwards is obtained directly 

 from the air itself. Hence the larger tribes of the vegetable 

 kingdom draw little from the soil, or rather excrete by their roots 

 more carbon than they absorb, whilst the extensive surface ex- 

 posed to the atmosphere by their leaves enables them to perform 



