622 



F A R IVI J: R S' REGISTER 



[No, 11 



is united by chemical aftraclion. Tiie moisture 

 which wood, or muscular fibre, or {rum, that have 

 been heated to 212'^, ati'ord by distillation at a red 

 heat, is likewise water, the elements of which 

 were united in the substance by chemical combi- 

 nation. 



When pipe-clay dried at the temperature of the 

 atmosphere is broujrht in contact with water, the 

 fluid is rapidly absorbed : this is owinir to cohesive 

 attraction. Soils in fj^eneral, vecretable and animal 

 substances, that have been dried at a heat below 

 that of boilino" water, increase in weiL^ht by expo- 

 sure 10 air, owinc to their absorbiri£r waterexislinfr 

 in the state of vapor in the air, in consequence o? 

 cohesive attraction. 



The water chemically combined nmonfrst the 

 elements of soils, unless in the case of the decom- 

 position of animal or vecretable substances, cannot 

 be absorbed by the roots of plants; but that ad- 

 hering to the parts of the soil is in constant use in 

 vegetation. Indeed, there are k\v mixtures of the 

 earths found in soils that contain any chemically 

 combined water ; water is expelled from the earths 

 by most substances that comliine with them. 

 Thus, if a combination of lime and water be ex- 

 posed to carbonic acid; the carbonic acid takes the 

 place of water ; and compounds of alumina and 

 silica, or other compounds of the earths, do not 

 chemically unite with water ; and soils, as it has 

 been stated, are formed either by earthy carbon- 

 ates, or compounds of the pure earths and metal- 

 lic oxides. 



When saline substances exist in soils, they may 

 be united to water both chemically and mechani- 

 cally ; but they are always in too "small a quantity 

 to influence materially the relations of the soil to 

 water. 



The power of the soil to absorb water by cohe- 

 sive attraction depends in great measure upon the 

 state of division of its parts; the more divided 

 they are. the greater is their absorbent power. 

 The different constituent parts of soils likewise 

 appear to act, even by cohesive attraction, with 

 difl'erent degrees of eneriry. Thus vegetable 

 substances seem to be more absorbent than ani- 

 mal substances ; animal substances more so than 

 compounds of alumina and silica; and compounds 

 of alumina and silica more absorbent than carbo- 

 nates of lim.e and magnesia: these differences 

 may, however, possibly depend upon the differ- 

 ences in their state of division, and upon the sur- 

 face exposed. 



The power of soils to absorb water from air is 

 much connected with fertility. When this power 

 is great, the plant is supplied with moisture in dry 

 seasons ; and the effect of evaporation in the day 

 is counteracted by the absorption of aqueous va- 

 por from the atmosphere, by the interior parts of 

 the soil during the day, and by both the exterior 

 and interior during the night. 



The stiff" clays approaching to pipe clays in their 

 nature, which take up the greatest quantity of 

 water when it is poured upon them in a fluid form, 

 are not the soils which absorb most moisiure from 

 the atmosphere in dry weather. They cake, and 

 present only a small surface to the air; and the 

 vegetation on them is generally burnt up almost 

 as readily as on sands. 



The soils that are most efficient in supplying the 

 plant with water by atmospheric absorption are 

 those in which there is a due mixture of sand. 



finely divided clay, and carbonate of lime, with 

 some anirnnl or ven;eiable matter; and which are 

 so loose and light as lo be freely permeable to the 

 atmosphere. With res[)ect to this quality, carbo- 

 nate of lime and animal and vegetable matter arc 

 of great use in soils, they irive absorbent power to 

 the soil without giving it likewise tenacity : sand, 

 which also destroys tenacity, on the contrary, gives 

 little absorbent power. 



I have compared the absorbent powers of many 

 soils with respect to atmospheric moisture, and I 

 have always found it greatest in the most fertile 

 soils; so that it aflx)rds one method of judging of 

 the firoductiveness of land. 



1000 parts of a celebrated soil from Ormistown, 

 in East Lothian, which (contained more than half 

 its weight of finely divided matter, of which 11 

 parts were carbonate of lime and 9 parts vegetable 

 matter, when dried at 212", gained in an hour by 

 exposure to air saturated with moisture, at tempe- 

 rature 62°, 18 grains. 



1000 parts of a very fertile soil I'rom the banks 

 of the river Parret, in Somersetshire, under the 

 same circumstances, gained 16 grains. 



1000 parts of a soil from JNlersea, in Essex, 

 worth 45 shillings an acre, gained 13 grains. 



1000 grains of a fine sand from Essex, worth 

 28 shillings an acre, gained 11 grains. 



1000 of a coarse sand, worth 15 shillings an 

 acre, crained only 8 grains. 



1000 of the soil of Bagshot heath gained only 

 3 grains. 



Water, and the decomposing animal and ve- 

 cretable matter existing in the soil, constitute the 

 true nourishment of plants: and as the earthy 

 parts of the soil are useful in retaining water, so 

 as to supply it in the proper proportions to the 

 roots of the vegetables, so they are likewise ef- 

 ficacious in producing the proper distribution of 

 the animal or vegetable matter: when equally 

 mixed with it, they prevent it from decomposing 

 too rapidly ; and by their means the soluble parts 

 are supplied in proper proportions. 



Besides this agency, which may be considered 

 as mechanical, there is another agency between 

 soils and organizable matters, which may be re- 

 garded as chemical in its nature. The earths^ 

 and even (he earthy carbonates, have a certain 

 deijree of chemical attraction for many of the 

 principles of vegetable and animal substances. 

 This is easily exemplified in the instance of alu- 

 mina and oil ; if an acid solution of alumina be 

 mixed wiih a solution of soap, which consists of 

 oily matter and potassa, the oil and the alumina 

 will unite and form a white powder, which will 

 sink to the bottom of the fluid. 



The extract from decomposing vegetable mat- 

 ter, when boiled with pipe-clay or chalk, forms a 

 combination by which the vegetable matter is 

 rendered more difficult of decomposition and of 

 solution. Pure silica and siliceous sands have 

 little action of this kind ; and the soils which con- 

 tain the most alumina and carbonate of lime are 

 those which act with the greatest chemical energy 

 in preservinii manures. Such soils merit the ap- 

 pellation which is commonly given to them of 

 rich soils ; for the vegetable nourishment is long 

 preserved in them, unless taken up by the organs 

 of plants. Siliceous sands, on the contrary, de- 

 serve the term hungry, which is commonly ap- 

 plied to them ; for the vegetable and animal mat- 



