Manures. 403 



dung will continue' to be used; it admits of clean workman- 

 ship, with less labour, and insures a good immediate crop : 

 to prevent loss as much as possible, therefore, the dung-heap 

 should be in a brick cistern, and covered over with earth at 

 least 9 in. deep, with a well at one corner to retain the drainage, 

 which, from time to time, should be returned over the heap. 



The chief component of plants is carbon, and we shall not 

 be far wrong if we estimate it as constituting 50 per cent, of every 

 vegetable ; it is the decayed organic remains of the soil which 

 supply a considerable proportion of this to the growing plants. 

 It is a subject of debate among chemists, how the carbon of 

 manures is imbibed by plants. Carbon, sa} r they, is insoluble, 

 and experiment has demonstrated that the roots cannot absorb 

 it in a solid state. Sennebier, having observed that water 

 impregnated with carbonic acid, when applied to the roots of 

 plants, was beneficial, concluded that the carbon of manures is 

 converted into carbonic acid, and is in that state imbibed by 

 them. [Phys. Veg., v. iii. p. 55.) 



Thomson, in an early edition of his System of Chemistry^ 

 gave a still more elaborate theory, which, being in subsequent 

 editions omitted, we have no necessity to demonstrate absurd. 

 I consider that the facts of which we are in possession, if pro- 

 gressively estimated, place the subject in a very clear light. 

 Saussure found that a soil deprived of its soluble matters, by 

 repeated decoctions with water, would not support vegetation 

 so well as that portion of the same soil not so deprived of its 

 soluble constituents. (Recherch. sur la Veg., cv. § ii. p. 170.) 

 The extract thus obtained was evidently composed of saccha- 

 rine matter, mucilage, extractive principle, &c. These, we 

 know, are nutritive to plants, and are elaborated and assimi- 

 lated by them after introsusception. Now, vegetable sub- 

 stances, as straw, &c, gradually yield these soluble matters as 

 they decay. Straw, wood, leaves, &c, consist chiefly of woody 

 fibres ; to convert this into saccharine and mucilaginous mat- 

 ters is the work of putrefaction ; to effect this, oxygen must be 

 absorbed, and the extra proportions of carbon be got rid of, 

 as is evident from the following* table of constituents. 





Woody Fibre. 



Gum. 



Sugar. 



Carbon 



52-53 



42-25 



27-5 



Oxygen 



41-78 



50-84 



64.7 



Hydrogen 



5-69 



6-95 



7*8 



100*00 100-00 100*0 



That such processes actually do occur, Saussure has de- 

 monstrated by experiment : he found that moist wood, exposed 



1) d 2 



