_Geol. and Nat. Hist. Survey of Minn._, XIX, Ann. Rept., p. 50. Thin sections of 2–3 sq. mm. area are of a convenient size, and they should be subjected to a red heat for (1½)-3 minutes. Too long a continuance of heat may render the sections too dark or lessen their transparency or produce melting. Footnote 141: This test may vary, in many cases graphite not being consumed even after long heating. Footnote 142: The isolation of material for investigation is of more interest for the lithologist or chemist than for the student of optical mineralogy; therefore only a very brief outline of some of the methods employed will be given. Footnote 143: The grains passing through different meshes are investigated microscopically to ascertain which size grains are homogeneous; the rest of the sample should then be reduced to grains of this size. Footnote 144: Further reference can be made to _Iddings’ Rock Minerals_, pp. 25 and 95, 1911. A convenient form of apparatus is also described in _School of Mines Quarterly_, Vol. X., p. 284, 1889. Footnote 145: S. L. Penfield, _Am. Jour. Sci._, Vol. L., p. 446, 1895. In this article a convenient form of separating apparatus is also described. Footnote 146: A convenient list of minerals, arranged in the order in which they would be attracted by increasing the force of the electro-magnet, is given in Weinschenk’s _Tabellen_. Footnote 147: For micro-chemical work the reagents should be applied in very small drops, which spread out on glass to discs 2 mm. in diameter. For manipulating the reagents use platinum wires, 0.5 mm. in thickness. Footnote 148: _Elemente einer neuen chemisch-mikroskopischen mineral- und Gesteins-analyse_, Pragg, 1877. Translation of above by Winchell in _Geol. and Nat. Hist. Survey of Minn._, Vol. XIX., Ann. Report, 1890. Footnote 149: The strength of the solution should be about 3½%; for if too weak many minerals do not give satisfactory results, and if too strong a very large number of fluosilicate crystals are formed together with the separation of much silica, thus making it impossible to carefully differentiate the crystals with a microscope. Footnote 150: As most of the rock-forming minerals that would be investigated are silicates, the hydrofluosilicic solutions can also be obtained by treatment with HFl. Footnote 151: After Lévy and Lacroix. Footnote 152: _Naturkunde_, Amsterdam, 2, Vol. XVII., 1881. _Chemical News_, Vol. LXIII., No. 1647, June 1891, et seq. _Micro-chemical Analysis_, Behrens (Judd), Macmillan & Co., London, 1894. Footnote 153: After Lévy and Lacroix. Footnote 154: After Lévy and Lacroix. Footnote 155: Compiled from Weinschenk’s Tables, as revised in _Petrographic Methods_ by Weinschenk-Clark, 1912. Footnote 156: Pirsson and Robinson, _Am. Jour. Sci._, iv, Vol. X, Oct., 1900. Footnote 157: Harker, _Petrology for Students_, p. 28, 1895.