EARTHS. 



EARTHS. 



The next important inquiry instituted by the 

 same excellent chemist, was the relative degree 

 of tenacity with which different soils retain the 

 moisture when exposed under similar circum- 

 stances to the action of the atmosphere ; and 

 he found that they parted with their moisture 

 according to the following rate: 



In these experiments the soils were spread 

 out to dry very thinly over a plate of metal ; 

 but in the following comparative trials (to ren- 

 der the results in all respects more similar to 

 those which the cultivator would experience), 

 the soil was exposed to the atmosphere in 

 masses of an inch in depth : 



The amount of the relative contraction of 

 different soils, when they are deprived of their 

 moisture, is another equally important question 

 to the farmer to be ascertained. " Many of 

 them," says M. Schubler, " become contracted 

 into a narrower space in drying, and in conse- 

 quence of this circumstance cracks and fissures 

 frequently occur in land, and have an injurious 

 effect on the vegetation, as the finer roots, which 

 often ramify horizontally, and not unfrequently 

 supply to the plants the greater part of their 

 means of nourishment, are, by such contrac- 

 tions, either laid bare of soil or torn asunder. 

 In order to subject soils to comparative expe- 

 riments on this point, the following plan may 

 be adopted. We either form of the earths, in 

 their wet state, large cubic pieces of equal 

 size, being at least ten-twelfths of an inch in 

 height, breadth, and length, or we let such 

 earths be fitted and dried one after another in 

 an accurately worked cubic inch ; after some 

 time, when the weight of these cubes of earth 

 ceases to change by further drying, we measure 

 the dimensions of the cube by means of a rule 

 on which the tenths of lines can be distin- 

 guished, and may thus calculate easily the 

 volume of the earth, and consequently ascer- 

 tain the diminution in bulk which has been 

 caused by the drying. The experiments which 

 I made with the following earths exhibited on 

 this point the subjoined differences: 

 432 



Such is the effect upon various soils of de- 

 riving them of their moisture. In these che- 

 nical investigations the farmer will see how 

 ntire;ly they confirm his own observations. 

 The heavy clay soils, he well knows, are the 

 most contracted by exposure to the heats of 

 summer; the sands the least affected of any. 



A still more important property of soils, 

 their attraction for the aqueous vapour of .the 

 atmosphere, is next to be considered a pro- 

 perty the importance of which to the cultiva- 

 tor, Sir H. Davy long since saw in its true 

 light, and his observations cannot be too often 

 quoted, since they well illustrate and enforce, 

 amongst other things, the truth of the great 

 Tullian system of agriculture: of the advan- 

 tages of finely dividing the soil, of the. subsoil 

 plough, and of the horse-hoe husbandry. "The 

 power of the soil to absorb water by cohesive 

 attraction," said this great chemist, "depends 

 in a great measure on the state of division of 

 its parts; the more divided they are, the great- 

 er is their absorbent power. The different 

 constituent parts of soils, likewise appear to 

 act, even by cohesive attraction, with different 

 degrees of energy: thus vegetable substances 

 seem to be more absorbent than animal sub- 

 stances, animal substances more so than com- 

 pounds of alumina and silica, and compounds 

 of alumina and silicia more absorbent than 

 carbonates of lime and magnesia; these dif- 

 ferences may, however, possibly depend upon 

 the differences in their state of division, and 

 upon the surface 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 

 vapour from the atmosphere by the exterior 

 parts of the soil during the night. The stiff 

 clays, approaching to pipe-clay 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 moisture 

 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 animal or vegetable matter; 

 and which are so loose and light as to be 

 freely permeable to the atmosphere. With 

 respect to this quality, carbonate of lime and 

 animal and vegetable matter are of great use 





