FORMATION OF NATURAL SOILS. 19 



water, rendering it muddy ; 3. organic matter, that is, vegetable and animal 

 remains, or humus. 



44. A closer, or chemical examination, requires that a fair sample of the 

 soil should be soaked in a glazed earthen basin, filled with rain or distilled 

 water, agitating it occasionally, so as to break any hard lumps of earth. 

 Where it is hard clay, rub the soil in a mortar with the pestle, adding water 

 from time to time till dissolved. Allow the whole to remain undistm-bed for a 

 few minutes, when the sand, stony fragments, and organic matter, will fall to 

 the bottom, by reason of their specific gravity ; the finer particles floating on 

 the water. This muddy water is to be poured off into a glass vessel, and the 

 deposit left washed repeatedly in clean water, till it comes off perfectly pure. 

 The residuum is reserved for further examination. 



45. The muddy water poured off first is suffered to remain at rest in the 

 vessel till the fine mud has quite settled at the bottom, and the water is 

 perfectly clean, when it is poured off also, and the water left to dry by 

 evaporation, in an earthen or porcelain vessel, 



46. The sand and gravel, the mud, and the residue of the pure liquid, into 

 which the soil has been separated, are mixed with organic matter. We can, 

 therefore, only separate the soluble from insoluble matter, and the finer vege- 

 table remains from the larger iDieces of roots and stems. The soluble organic 

 matter remains in solution, the finer portions with the mud, and the coarser 

 with the gravel and sand. By heating a portion to red heat, in an iron spoon 

 or platinum dish, the organic matter will first blacken the soil and then dis- 

 appear entirely ; when the heat is continued some time and increased, the in- 

 combustible matter being left behind, is generally coloured red by the oxide of 

 iron, which is present in every soil. The organic part which burns is called the 

 combustible part. Thus : — 1. stony fi-agments ; 2. impalpable powder ; 3. 

 soluble organic and inorganic matter, are obtained by the washing process. 



47. The sand and gravel vary much in character, partaking of the rocks 

 from which they emanate. 



48. The impalpable powder will be found a mixture of clay, with very fine 

 fragments of stone and gravel, and organic matter. In it the chemist 

 will distinguish alumina combined with silica, free alumina, free sihca ; oxide 

 of iron, of manganese, of lime, of magnesia, of potash, and soda ; with traces 

 of phosphorus, sulphuric and carbonic acid ; silica, or sand, predominating. 



49. Organic matter is recognized by the black colour the powder assumes 

 when heated over a spirit-lamp. The watery solution evaporated to dryness, 

 leaves an inconsiderable residue, generally coloured brown by organic matter, 

 which may be drawn off by heat. In the combustible residue, chemical tests 

 will generally discover ammonia, humic, ulmic, cremic, and apocrenic acids, 

 all known under the common name of humus. In the incombustible, potash, 

 soda, lime, magnesia, phosphoric acid, sulphuric acid, silicic acid, chlorine, and 

 occasionally oxide of iron and manganese will be found, with nitre, iodine, and 

 bromine ; this latter, however, of very rare occux'rence, and only in soils near 

 the sea or near to salt-springs. 



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