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of salt solutions of different degrees of concentration, which states that the amount of solute which diffuses through a given cross-section is proportional to the difference in concentration of two cross-sections lying indefinitely near to each other, or is proportional to the difference in cross-section. From which it is seen that the fineness of the raw materials governs the conduct of the body, as the area of the surface of the particles in contact is dependent thereon. Mechanical analysis is complicated by the fact that the raw materials differ only slightly in specific gravity, as appears in this table : Kaolinite 2.6 Calcite 2.7 Biotite 2.7-3.1 Quartz 2.6 Felspars 2 . 55-2 . 75 Micas 2.7-3.0 The methods in use depend therefore on variety of size and shape of the particles, and not on their density or mineral composition. In all cases the clay must first be deflocculated, and a few drops of ammonia or alcohol is probably the best agent for this purpose. Schloessing (1903) recommends preliminary extraction with weak acid, and then treatment with ammonia after washing free from acid. If all colloids are present as alkaline compounds, shaking with cold distilled water alone is