By far the most convenient liquid for ordinary use is methylene iodide, which has already been recommended for its high refraction. It has, when pure, a density at ordinary room-temperatures of 3·324, and it is miscible in all proportions with benzol, whose density is O·88, or toluol, another hydrocarbon which is somewhat less volatile than benzol, and whose density is about the same, namely, 0·86. When fresh, methylene iodide has only a slight tinge of yellow, but it rapidly darkens on exposure to light owing to the liberation of iodine which is in a colloidal form and cannot be removed by filtration. The liquid may, however, be easily cleared by shaking it up with any substance with which the iodine combines to form an iodide removable by filtration. Copper filings answer the purpose well, though rather slow in action; mercury may also be used, but is not very satisfactory, because a small amount may be dissolved and afterwards be precipitated on to the stone under test, carrying it down to the bottom of the tube. Caustic potash (potassium hydroxide) is also recommended; in this case the operation should preferably be carried out in a special apparatus which permits the clear liquid to be drawn off underneath, because water separates out and floats on the surface. In Fig. 32 three cut stones, a quartz (_a_), a beryl (_b_), and a tourmaline (_c_) are shown floating in a diffusion column of methylene iodide and benzol. Although the beryl is only slightly denser than the quartz, it floats at a perceptibly lower level. These three species are occasionally found as yellow stones of very similar tint. [Illustration: FIG. 32.—Stones of different Specific Gravities floating in a Diffusion Column of heavy Liquid.] Various other liquids have been used or proposed for the same purpose, of which two may be mentioned. The first of them is a saturated solution of potassium iodide and mercuric iodide in water, which is known after the discoverer as Sonstadt’s solution. It is a clear mobile liquid with an amber colour, having at 12° C. a density of 3·085; it may be mixed with water to any extent, and is easily concentrated by heating; moreover, it is durable and not subject to alteration of any kind; on the other hand, it is highly poisonous and cauterizes the skin, not being checked by albumen; it also destroys brass-ware by amalgamating the metal. The second is Klein’s solution, a clear yellow liquid which has at 15° C. a density of 3·28. It consists of the boro-tungstate of cadmium, of which the formula is 9WO_{3}.B_{2}O_{3}.2CdO.2H_{2}O + 16Aq, dissolved in water, with which it may be diluted. If the salt be heated, it fuses at 75° C. in its own water of crystallization to a yellow liquid, very mobile, with a density of 3·55. Klein’s solution is harmless, but it cannot compare for convenience of manipulation with methylene iodide.