The grinding apparatus consists of two circular disks of solid copper, nine inches in diameter, and three-eighths inch thick, which may be used alternately as different grades of emery are required. They are attached either by a screw or square socket to a vertical iron spindle which revolves smoothly in a conical bearing. The grinding disk is surrounded when in use by a large cylindrical pan of tin, which is not shown in the cut, which has an opening in its center to allow of the passage of the spindle. The sawing apparatus consists of a horizontal countershaft placed on a different part of the table and connected with the motor by a separate belt. It carries at one end a vertical wheel of solid emery, and at the other an attachment, level-table and guide for the diamond-saw. A small water-can with spout, not shown in the cut, is suspended over the edge of the table to keep the saw wet when it is in use. The machine is very conveniently driven by a storage battery when street circuits cannot be drawn on. For the details of making mineral sections, the works on petrography may be consulted. =262. Separation of Silt Particles by Specific Gravity Solutions.=—In silt separates the specific gravity of the different mineral particles present may vary from graphite (1.9–2.3) to hematite (5.2–5–3). The following list gives the specific gravities of some of the more common minerals which may be met with in soils: Gypsum 2.31 Albite 2.56–2.63 Quartz 2.65 Talc 2.74 Chlorite 2.78 Muscovite 2.85 Calcite 2.5–2.78 Dolomite 2.90 Tourmaline 2.94–3.3 Biotite 3.01 Apatite 3.16 Pyroxenes 3.22–3.5 Epidote 3.39 Titanium Minerals 3.48–4.75 Iron oxids 5.2–5.3 The finest particles of silt are separated by gravity with great difficulty, inasmuch as they tend to remain suspended in the solutions for an indefinite period. With the coarser silts, however, useful data are often obtained by this method. The separation is preceded by extraction of the particles with hydrochloric acid to remove encrusted soluble matter, and by ignition to destroy any traces of organic matter. Those mineral matters which are soluble in acid or are changed by ignition must, of course, be sought for in separate portions of the silt, =263. Thoulet’s Solution.=[168]—The standard solution is of such a density that particles of 2.65 specific gravity-will just float thereon, using for this purpose a solution of about 2.7 specific gravity. The solution from which the above standard is prepared is made as follows: