Heat and its counter agent frost are the most powerful forces in nature, their sensible physical effects being the expansion and contraction of matter.
Water has two modes of action, physical and chemical. This agent is the great destroyer of the important forces, cohesion and friction. Cohesion is a force uniting particles of matter and resists their separation when the motion attempted is perpendicular to the plane of contact. Friction is a force resisting the separation of surfaces when motion is attempted which produces sliding. The hydrostatic pressure and resultant effect upon submerged surfaces need to be kept constantly in mind. When the surface is impermeable the line of pressure is normal to its plane, but when once saturated there are also horizontal and vertical lines of pressure. Since the strength of an earth dam depends upon two factors, namely, its weight and frictional resistance to sliding, the effect of water upon different materials entering into an earth structure should be most carefully considered. This will therefore occupy a large place in these pages. An earth embankment founded upon rock may become saturated by water forced up into it from below through cracks and fissures, reducing its lower stratum to a state of muddy sludge, on which the upper part, however sound in itself, would slide. The best preliminary step to take in such a case is to intersect the whole site with wide, dry, stone drains, their depths varying according to the nature of the ground or rock.
Air contains two ingredients ever active in the process of decomposition, carbonic acid and oxygen.
Organic Life accomplishes its decomposing effect both by physical and chemical means. The effect of organic matter upon the mineral ingredients of the soil may be stated as follows:
1. By their hydroscopic properties they keep the soil moist.
2. Their decomposition yields carbonic acid gas.
3. The acids produced disintegrate the mineral constituents, reducing insoluble matter to soluble plant food.
4. Nitric acid results in nitrates, which are the most valuable form of nutritive nitrogen, while ammonia and the other salts that are formed are themselves direct food for plants.
Vegetable Humus is not the end of decomposition of organic matter, but an intermediate state of transformation. Decay is a process almost identical with combustion, where the products are the same, and the end is the formation of water and carbonic acid, with a residue of mineral ash. The conditions essential to organic decomposition are also those most favorable to combustion or oxidation, being (1) access of air, (2) presence of moisture, and (3) application of heat.
Now the coöperation of these chemical and physical forces, which are ever active, is called “weathering.” Slate rock, for instance, weathers to clay, being impregnated with particles of mica, quartz, chlorite and hornblend. Shales also weather to clay, resulting often in a type of earth which is little more than silicate of aluminum with iron oxide and sand.