_Phosphorus._—When a mineral fragment containing phosphorus is treated according to the usual analytical methods for securing the ammonium magnesium phosphate, crystals are obtained of the form shown in Figs. 54 and 55. A phosphatic fragment of silt may be identified when soluble by treatment with nitric acid and ammonium molybdate. On slowly drying, rhombohedral crystals are produced, yellow by reflected, and green by transmitted light. Their form is shown in Fig. 56. =260. Petrographic Examination of Silt Particles.=—The larger silt particles and the minute fragments of minerals in the soil can best be studied in thin sections. For this purpose the following plan, proposed by Thoulet, may be used. Mix the soil minerals in considerable proportion—Thoulet recommends ten per cent, but a greater percentage is often better—with zinc oxid and make into a paste with sodium silicate. The paste should be worked to the consistence of putty and then rolled into little tablets about one-eighth of an inch thick and an inch in diameter. After drying a day or two without heating, the tablets become hard enough to mount and grind like rock sections. These tablets are mounted in Canada balsam on glass slides and ground as thin as possible with fine emery on the turn-table or glass plate, as rock sections are treated. As these tablets are not as strong as rock sections usually are, they require care in this treatment. Some of the grains also are apt to be torn out in the process of grinding and to compensate for this loss a number of slides should be prepared with each lot of soil minerals. When this operation has been successful, the optical properties of the various minerals can be studied as in rock sections. As the iron oxid contained in the soils obscures the transparency of the minerals, it is well to treat a portion of the material under examination with hot hydrochloric acid for a short time to remove this oxid and then prepare slides with the cleansed material and compare results with the untreated. As the acid will dissolve phosphates and carbonates, and will partly or wholly decompose some other minerals, the operator must be guided by his judgment in its use. =261. Machine for Making Mineral Sections.=—A convenient apparatus for this purpose has been described by Williams[167] and is represented in Fig. 57. It is supported on a substantial table provided underneath with electric batteries and a motor for driving the cutting disks seen on the top. The table is three feet six inches square and two feet nine inches high. [Illustration: FIGURE 57. MACHINE FOR MAKING MINERAL SECTIONS. ]