Book III. ALTERATION OF THE PARTS OF THE SOIL. 



327 



wall, through which the above 

 water is conveyed into the buc- 

 kets ; the other trough is hid by 

 two stone walls that support the 

 wheel : d dd, buckets, of which 

 80 are arranged on each side of 

 the arms of the wheel, in all 160 ; 

 e e e, Si cistern, into which the 

 water raised by the buckets is 

 discharged ; ///, wooden barrel 

 pipes, through which the water 

 descends from the cistern under 

 ground. 



2190. The cistern of the Blair- 

 Drummond wheel, as seen from 

 above [Jig. 205.), shows the two 

 troughs into which the buckets 

 empty themselves [gg) ; the space 

 through which the water flows to 

 the barrel pipes (// in fig. 204.) 

 (A) ; the place where the arms of 

 the wheel move (i), and where the float boards and buckets descend (A). The buckets are filled from 



two side troughs {fig. 206. /), which communicate 

 ^^ with the head of water which drives the wheel, as 



206 ^Sfc. seen at e in j?^. 204. (Fa?vw. Ma^., vol. xviii.) 



2191. Incineration. The chemical changes 

 which can be effected in soils by inciner- 

 ation are considerable. This practice was 

 known to the Romans, is more or less in use 

 in most parts of Europe, is mentioned as an 

 approved practice by our oldest agricultural 

 writers, and has lately excited some degree 

 of attention from the successful experiments 

 of different cultivators. {Far?ner's Magazine, 

 1810 to 1815, and Farmer s Journal, 1814 

 to 1821 ) 



2 1 92. The theory of burning soils is thus 

 given by Sir H. Davy. It rests, he says, 

 entirely on chemical doctrines. The bases of all common soils are mixtures of the pri- 

 mitive earths and oxide of iron ; and these earths have a certain degree 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 common silicious stone. Feldspar, for instance, contains 

 silicious, aluminous, and calcareous earths, fixed alkali, and oxide of iron, which exist in 

 one compound, in consequence of their chemical attractions for each other. Let this 

 stone be ground into impalpable powder, it then becomes a substance like clay ; if the 

 powder is heated very strongly, it fuses, and on cooling forms a coherent mass similar to 

 the original stone ; the parts separated by mechanical division adhere again in conse- 

 quence of chemical attraction. If the powder be heated less strongly, the particles only 

 superficially 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 is estimated, it is 

 foimd much less in the latter case. The same effect takes place when the powder 

 of other silicious or aluminous stones is made the subject of experiment ; and two equal 

 portions of basalt ground into impalpable powder, of which one half 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 exposed to air. In four hours the 

 one had gained only two grains, whilst the other had gained seven grains. When clay 

 or tenacious soils are burnt, the effect is of the same kind ; they are brought nearer to a 

 state analogous to that of sands. In the manufacture of bricks the general principle is 

 well illustrated ; if a piece of dried brick earth be applied to the tongue, it will adhere to 

 it very strongly, in consequence of its power to absoib water ; but after it has been burnt, 

 there will be scarcely a sensible adhesion. 



21 93. The advantages of burning are, that it renders the soil less compact, less tenacious, 

 and less retentive of moisture ; and when properly applied, may convert a matter which 

 was stiff, damp, and, in consequence, cold, into one powderj', dry, and warm, and much 

 more proper as a bed for vegetable life. 



2194. The great objection made by speculative chemists to paring and burning is, that 

 it destroys vegetable and animal matter, or the manure in soil : but in cases in which the 

 texture of its earthy ingredients is permanently improved, there is more than a compen- 

 sation for this temporary disadvantage ; and in some soils where there is an excess of inert 

 vegetable matter, the destruction of it must be beneficial ; and the carbonaceous matter 

 remaining in the ashes may be more useful to the crop than the vegetable fibre from 

 which it was produced. 



2195. Three specimens of ashes from different lands which had luidergone paring and 



Y 4 



