AGRICULTURAL CHEMISTRY. 149 



soils which are filled with pebbles or boulders, that were then 

 rounded and polished in their transit from distant nothern latitudes. 



Since the opening of the human epoch, lesser local floods in the 

 waters of rivers have made numberless so-called alluvial deposits 

 (river bottoms and deltas) in like manner. 



Changes of temperature, especially the alternate freezing and thaw- 

 ing of water, exercise great influence in the pulverization of rocks. 

 Water, as is well known, expands with great force in the act of freez- 

 ing, and by insinuating itself while liquid into the fine cleavage rifts 

 of rocks, and there congealing, breaks asunder the particles. The 

 dense limestone of the Jura formation, as found in polished nodules 

 in the soil near Munich, in Bavaria, if moistened with water and ex- 

 posed to frost a single night is so disintegrated that, as the ice melts, 

 it yields a water tuibicl with the loosened atoms of rock. 



Oxygen exerts a perpetual disintegrating effect, by uniting with 

 the protoxyd of iron, which occurs in nearly all rocks, setting free 

 the acids and bases before in combination with it, and yielding 

 peroxyd of iron. Sulphid (sulphuret) of iron is an exceedingly 

 abundant ingredient of rocks, and, under the influence of oxygen, is 

 readily converted into soluble sulphate of iron — a product which, in 

 turn, reacts upon other constituents of rocks to dissolve or alter them. 

 Carbonic acid, especially in conjunction with water, dissolves or com- 

 bines with the alkalies and earths existing in rocks, and thus destroys 

 their integrity and causes them to crumble away to soil. 



The composition and chemical characters of soils depend upon the 

 kind of rock or rocks from which they originate. A glance at the 

 nature of these will therefore be of service to us. As to chemical 

 ingredients, we find that the most abundant and widely diffused are 

 precisely those which are found in the ash of plants. They mostly 

 occur in certain definite combinations, and form the minerals, quartz, 

 feldspar, hornblende, augite, mica, serpentine, kaolin, zeolite, carbo- 

 nate of lime, carbonate of magnesia, and numerous others of less im- 

 portance. The composition of specimens of these minerals is given 

 in the annexed table. They occur, however, in very numerous varie- 

 ties, and vary greatly in the kind as well as proportions of their in- 

 gredients.* It is seen from the table that many of them contain 

 nearly all the inorganic ingredients of plants. 



e This fact may appear to stand in contradiction to the statement above made that these 

 minerals are definite combinations. In the infancy of mineralogy great perplexity arose from 

 the numberless varieties of minerals that were found — varieties that agreed together in cer- 

 tain characteristics, but widely differed in others. In 1830, Mitscherlich, a Prussian phi- 

 losopher, discovered that a number of the elementary bodies are capable of replacing each 

 other in combination, from the fact of their natural crystalline form being identical; they 

 being, as he termed it, isomorphous, or of like shape. Thus, magnesia, lime, protoxyd of iron, 

 and protoxyd of manganese; potash, soda; silica, and alumina may replace each other in 

 such a way as to greatly affect the composition without altering the constitution of a 

 mineral. Of the mineral hornblende, for example, there are known a great number of 

 varieties ; some pure white in color, containing, in addition to silica, magnesia and lime ; 

 others pale green, a small portion of magnesia being replaced by protoxyd of iron ; others 

 black, containing alumina in place of a portion of silica, and with oxides of iron and man- 

 ganese in large proportion. All these minerals, however, admit of one expression of their 

 constitution, for the amount of oxygen in the bases, no matter what they are, or what their 

 proportions, bears a constant relation to the oxygen of the silica (and alumina) they con- 

 tain, the ratio being 4 : 9. 



