rOBMATION OF NATURAL SOILS. 21 



to the filtered solution; the precipitate is aluminum in combination with 

 water ; and, if iron is present, it will be precipitated in the state of red oxide 

 along with the alumina, which is to be evaporated and collected also on a filter, 

 and the filtering-paper carefully washed as before. 



54. Having weighed each of the filtrates, the filters themselves and the 

 crucibles are to be carefully dried, and finally severally ignited in a platinum 

 crucible, and the amount of ashes yielded by the filter deducted in each case 

 from the aggregate weight of the filter and precipitates ; but if iron is present, 

 it will be necessary first to separate the iron from the alumina by adding to 

 the last filtrate a solution of caustic potash, which dissolves the alumina, leav- 

 ing the oxide of iron untouched. This is to be accurately collected, washed 

 by a process of decantation, and heated to a red heat, cooled, and weighed ; 

 while the alumina, which we left dissolved in potash, is to be treated with 

 the nitrate of ammonia, boiled, and collected by filtration, heated to redness, 

 and finally weighed. 



55. Soils, it is evident, are due to the disintegration of the solid rock, which 

 has been going on for thousands of years ; in the course of which time the 

 surface of the country has thus been covered by a coating of disintegrated rock, 

 varying in depth and in character with the mineral nature of the neigh- 

 bourhood. Sandstone has produced a light, porous, sandy soil ; slaty shale 

 has yielded a stiff, cold, impervious clay ; from the crumbUng limestone a 

 calcareous soil has been formed ; and the trap-rock of the primitive forma- 

 tion has yielded a rich, fertile, and generally reddish-grey loam. Basalt rock, 

 which prevails over great part of Scotland and the north of Ireland, and here 

 and there in England, gives a friable fertile soil, also of reddish-grey ; while 

 the soils resting on the chalk formation generally partake of a dry, loose, 

 friable character, congenial to many of the most useful forms of vegetation. 

 While useful to the gardener, an intimate knowledge of the characters of 

 these soils is essential to the farmer. To a large extent, the gardener, ope- 

 rating on a limited scale, can prepare his soils, and ameliorate their nature, 

 by rule-of-thumb admixture, and by the use of humus, or vegetable mould, 

 the product of decomposed animal and vegetable manures. Of all the con- 

 stituents we have named, humus performs the most important part in the 

 direct food for the nutrition of plants ; but whether it combines with organic 

 matter and forms plant, or whether it only exercises a beneficial influence on 

 vegetation by furnishing a continual source of carbonic acid by its decomposi- 

 tion, or by condensing ammoniac gas from the atmosphere, is by no means a 

 settled question, the best chemists differing widely on the point ; some of them 

 denying altogether the efficacy of inorganic matter in soils. Recent experiments, 

 however, show distinctly the great influences that inorganic matter exercises 

 over the growth of plants : it is taken up by the roots, and may be traced in 

 the ashes of plants ; and it has been most satisfactorily proved, that organic 

 matter alone is incapable of supplying all the wants of the growing plant, 

 certain inorganic substances being required by every plant, which, if not 

 present in the soil, there is a barrier to its healthy growth. There can be no 



