Distinction between haüynite (contains CaSO_{4}) and noselite (contains Na_{2}SO_{4}). Treat with HCl and on evaporation the characteristic crystals of gypsum will be seen if the mineral is haüynite. Dilute acid should be used and as low a temperature maintained as possible, otherwise crystals of anhydrite would form instead of gypsum. Recognition of apatite by test for phosphorus. Treat with a drop of ammonium molybdate dissolved in HNO_{3}. After complete action remove the solution to a clean object glass, when after slight warming a large number of very small yellow crystals (rhombic dodecahedral in shape) will form. The test may be used to distinguish this mineral from nephelite, melilite and natrolite. In the presence of soluble silica evaporate to render it insoluble and treat again with HNO_{3} and the reagent. Other micro-chemical tests are not mentioned for the reason that in elaborate chemical investigations of sections or isolated fragments recourse should be made to the most complete publications on the subject. APPENDIX. BRIEF SCHEME OF CLASSIFICATION INTO SYSTEMS BY OPTICAL DETERMINATIONS. ▄HOMOGENEOUS.▄ The whole substance shows the same optical character, except in the case of twin crystals when the different portions of the twin are affected differently. ▄Isotropic▄ All sections of the substance remain dark during a complete rotation between crossed nicols, and no interference figure is produced by convergent light. Amorphous Absence of crystalline form or cleavage. Isometric Presence of crystalline form or cleavage. ▄Anisotropic.▄ Sections generally show some interference color and extinguish four times, at 90° apart, during complete rotation. _Uniaxial._ Determined by character of interference figures obtained by convergent light from sections which remain dark or nearly so during complete rotation. All sections show parallel or symmetrical extinction. Tetragonal Sections giving interference figures are four- or eight-sided, or show rectangular cleavage. Hexagonal Sections giving interference figures are three-, six- or nine-sided, or show cleavage lines intersecting at 60°. _Biaxial_ Determined by character of interference figures obtained by convergent light. Orthorhombic Extinction is parallel or symmetrical in all sections parallel to _ă_, [_=b_] and _ć_. Color distribution is symmetrical to two lines and to the central point, see p. 48.