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. In the vicinity of the Tabeaud Dam, recently built under the personal supervision of the author, the construction of which will be described later, there is to be found a species of potash mica, which in decomposing yields a yellow clay (being ochre-colored from the presence of iron), mixed with particles of undecomposed mica. This material is subject to expansion, and by reason of its lack of grit and its unctuous character it was rejected or used very sparingly. Analysis of this material gave, Silica, 54.1 to 59.5%; potash, 1.5 to 2.3%; soda, 2.7 to 3.7%. Soil analysis may be either mechanical or chemical. For purposes of earthwork, we are most interested in the former, having to deal with the physical properties of matter. Chemical analysis, however, will often afford information of great value regarding certain materials entering into the construction of earth dams. The most important physical properties are: (1) Weight and specific gravity. (2) Coefficient of friction and angle of repose. (3) Structure and coloring ingredients. (4) Behavior toward water. There are two distinct methods of mechanical analysis: (1) Granulating with sieves, having round holes. (2) Elutriating with water, the process being known as silt analysis. It would require a large volume to present the subject of soil analysis in any way commensurate with its importance. Experiments bearing upon the subjects of imbibition, permeability, capillarity, absorption and evaporation, of different earth materials, are equally interesting and important.[1] The permeability of soils will be discussed incidentally in connection with certain infiltration experiments to be given later.