1838] 



FARMERS' K E G I S T E R . 



657 



Vir<ril in the first book of the Georgics: "Sa^pe I of inert vegetable matter, the destruction of it 

 etiam steriles incenderc protuit agros." It in a must be beneficial : and the carbonaceous matter 

 practice still much in use in many pans ol these j remaining in the ashes may be more useflil to the 

 islands; the theory of its operation has occasion- 1 crop than the vegetable fibre from which it was 

 ed much discussion, both among scieiUilic men produced. 



and farmers. It rests entirely upon chemical doc- 1 I have examined by a chemical analysis three 

 trines; and I trust I shall be able to offer you satis- specimens of ashes, from different lands that had 

 fiictory elucidations on the subject. j undergone paring and burning. The first was a 



The bases of all common soils, as I stated in the quantity sent to the board by Mr. lioys of Bell- 



fourth lecture, are mixtures of the primitive earths 

 and oxide of iron; and these earths have a certain 

 desree of attraction for each other. To regard 

 this attraction in its proper point of view, it is only 

 necessary to consider the composition of any com- 

 h)on siliceous stone. Feldspar, for instance, con- 

 tains siliceous, aluminous, calcareous earths, fixed 

 alkali, and oxide of iron, which exist in one com- 

 pound, in consequence of their chemical attrac- 

 tions for each other. Let this stone be ground 

 into impalpable powder, it then becomes a sub- 

 stance like clay: if the povvder be heated very 

 strongly it fiises, and on cooling forms a coherent 

 mass similar to the original stone; the parts sepa- 

 rated by mechanical division adhere again in con- 

 sequence of chemical attraction. If the powder 

 is heated less strongly, the particles only superfi- 

 cially combine with each other, and form a gritty 

 mass, which, when broken into pieces, has the 

 characters of sand. 



If the power of the powdered feldspar to absorb 

 water from the atmosphere, before and after the 

 application of the heat, be compared, it is found 

 much less in the last case. 



The same effect takes place when the powder 

 of other siliceous or aluminous stones is made the 

 subject of experiment. 



I found that two equal portions of basalt ground 

 into impalpable powder, of which one had been 

 strongly ignited, and the other exposed only to a 

 temperature equal to that of boiling water, gained 

 very different weights in the same time when ex- 

 posed to air. In four hours the one had gained only 

 two grains, whilst the other had sained seven grains. 

 When clay or tenacious soils are burnt, the ef- 

 fect is of the same kind; they are brought nearer 

 to a state analogous to that of sands. 



In the manufiicture of bricks the general princi- 

 ple is well illustrated; if a piece of dry brick earth 

 be applied to the tongue, it will adhere to it very 

 strongly, in consequence of its power to absorb 

 water; but after it has been burnt there will be 

 scarcely a sensible adhesion.* 



The process of burning renders the soil less 

 compact, less tenacious and retentive of moisture ; 

 and when properly applied, may convert a matter 

 that was stiff, damp, and in consequence cold, into 

 one powdery, dry, and warm ; and much more 

 proper as a bed lor vegetable life. 



The great objection made by speculative che- 

 mists to paring and burning is, "that it destroys ve- 

 getable and animal matter, or the manure "in the 

 soil ; but in cases in which the texture of its earthy 

 ingredients is permanently improved, there is more 

 than a compensation for this temporary disadvan- 

 tage. And in some soils where there is an excess 



* Clay has the remarkable property of confining 

 water; and therefore a thin layer of clay may have a 

 very injurious effect, by preventing rain water from 

 penetrating deeply and saturating the subsoil; an evil 

 which may be corrected by paring and burning.— J. D. 



hanger, in Kent, whose treatise on paring and 

 burning has been published. They were from a 

 chalk soil, and 200 grains contained 

 80 Carbonate of lime. 

 11 Gypsum. 

 9 charcoal. 

 15 Oxide of iron. 



3 Saline matter; viz. sulphate of potash, 

 muriate of magnesia, with a minute 

 quantity of vegetable alkali. 

 The remainder alumina and silica. 



Mr. Boys estimates that 2660 bushels are the 

 common produce of an acre of ground, which, ac- 

 cordirig to his calculation, would give 172900 lbs, 

 containing 



Carbonate of lime - 69160 /6s. 

 Gypsum - - 9509-5 



Oxide of iron - 12967'5 



Saline matter - 25935 



Charcoal - - 7780-5 



In this instance there was undoubtedly a very 

 considerable quantity of matter capable of being 

 active as manure produced in the operation of 

 burning. The charcoal was very finely divided ; 

 and, exposed on a large surface on the field, must 

 have been gradually converted into carbonic acid. 

 And gypsum and oxide of iron, as I mentioned in 

 the last lecture, seem to produce the very best ef- 

 fects when applied to lands containing an excess 

 of carbonate of lime. 



The second specimen was from a soil near 

 Coleorton, in Leicestershire, containing only four 

 per cent, of carbonate of lime, and consistino- of 

 three-fourths light siliceous sand, and about one- 

 fourth clay. This had been turf before burning, 

 and 100 parts of the ashes gave 



6 Parts charcoal. 

 3 Muriate of soda and sulphate of potash, 



with a trace of vegetable alkali. 

 9 Oxide of iron. 

 And the remainder the earths. 



In this instance, as in the other, finely divided 

 charcoal was found ; the solubility of which would 

 be increased by the presence of the alkali. 



The third instance was that of a stiff clay, from 

 Mount's Bay, Cornwall. This land had been 

 brought into cultivation from a heath by burning 

 about ten years before ; but having been neglect° 

 ed, furze was springing up in different parts of it, 

 which gave rise to the second paring and burning : 

 100 parts of the ashes contained 

 8 Parts of charcoal. 



2 Of saline matter, principally common 

 salt, with a little vegetable alkali. 



7 Oxide of iron." 

 2 Carbonate of lime. 



Remainder alumina and silica. 



Here the quantity of charcoal was greater than 



in the other instances. The salt, 1 suspect, was 



ovying to the vicinity of the sea, it being but two 



miles off. In this land there was certainly an ex- 



'cess of dead vegetable fibre, as well as unprofita- 



