(5) Cystin crystals are colorless, highly refractive, rather thick, hexagonal plates with well-defined edges. They lie either singly or superimposed to form more or less irregular clusters (Fig. 36). Uric acid sometimes {109} takes this form and must be excluded. Cystin is soluble in hydrochloric acid, insoluble in acetic; it is readily soluble in ammonia and recrystallizes upon addition of acetic acid. Cystin crystals are very rare, and when found, point to cystin calculus. (6) Fat-globules.--Fat appears in the urine as highly refractive globules of various sizes, frequently very small. These globules are easily recognized from the fact that they are stained black by osmic acid and red by Sudan III. The stain may be applied upon the slide, as already described (p. 103). Osmic acid should be used in 1 per cent. aqueous solution; Sudan III in saturated solution in 70 per cent. alcohol. Fat in the urine is usually a contamination from unclean vessels, oiled catheters, etc. A very small amount may be present after ingestion of large quantities of cod-liver oil or other fats. In fatty degeneration of the kidney, as in phosphorus-poisoning and chronic parenchymatous nephritis, fat-globules are commonly seen, both free in the urine and embedded in cells and tube-casts. In _chyluria_, or admixture of chyle with the urine as a result of rupture of a lymph-vessel, minute droplets of fat are so numerous as to give the urine a milky appearance. Chyluria occurs most frequently as a symptom of infection by _Filaria sanguinis hominis_. 2. In Alkaline Urine.--(1) Phosphates.--While most common in alkaline urine, phosphates are sometimes deposited in neutral or feebly acid urines. The usual forms are: (_a_) Ammoniomagnesium phosphate crystals; (_b_) acid calcium phosphate crystals; and (_c_) amorphous phosphates. All are readily soluble in acetic acid. (_a_) _Ammoniomagnesium Phosphate Crystals_.--They are {110} the common "triple phosphate" crystals, which are generally easily recognized (Figs. 37 and 63, and Plate IV). They are colorless, except when bile-stained. Their usual form is some modification of the prism, with oblique ends. Most typical are the well-known "coffin-lid" and "hip-roof" forms. The long axis of the hip-roof crystal is often so shortened that it resembles the envelop crystal of calcium oxalate. It does not, however, have the same luster; this, and its solubility in acetic acid, will always prevent confusion. [Illustration: FIG. 37.--Various forms of triple-phosphate crystals (Ogden).] When rapidly deposited, as by artificial precipitation, triple phosphate often takes feathery, star- or leaf-like forms. These gradually develop into the more common prisms. X-forms may be produced by partial solution of prisms.