(3) Calcium Oxalate.--Characteristic of calcium oxalate are colorless, glistening, octahedral crystals, giving the appearance of small squares crossed by two intersecting diagonal lines--the so-called "envelop crystals" (Fig. 47). They vary greatly in size, being sometimes so small as to seem mere points of light with medium-power objectives. Unusual forms, which, however, seldom occur except in conjunction with the octahedra, are colorless dumb-bells, spheres, and variations of the octahedra (Fig. 34). The spheres might be mistaken for globules of fat or red blood-corpuscles. Crystals of calcium oxalate are insoluble in acetic acid or caustic soda. They are dissolved by strong hydrochloric acid, and recrystallize as octahedra upon addition of ammonia. They are sometimes encountered in alkaline urine. [Illustration: FIG. 34.--Various forms of calcium oxalate crystals (Ogden).] The crystals are commonly found in the urine after ingestion of vegetables rich in oxalic acid, as tomatoes, {107} spinach, asparagus, and rhubarb. They have no definite significance pathologically. They often appear in digestive disturbances, in neurasthenia, and when the oxidizing power of the system is diminished. Like uric acid, their chief clinical interest lies in their tendency to form calculi, and their presence in fresh urine, together with evidences of renal or cystic irritation, should be viewed with suspicion, particularly if they are clumped in small masses. (4) Leucin and Tyrosin.---Crystals are deposited only when the substances are present in considerable amount. When present in smaller amount, they will usually be deposited if a drop of the urine be slowly evaporated upon a slide. They generally appear together, and are of rare occurrence, usually indicating severe fatty destruction of the liver, such as occurs in acute yellow atrophy and phosphorus-poisoning. The crystals cannot be identified from their morphology alone, since other substances, notably calcium phosphate (Fig. 38) and ammonium urate, may take similar or identical forms. _Leucin crystals_ (Fig. 35) are slightly yellow, oily-looking spheres, many of them with radial and concentric {108} striations. Some may be merged together in clusters. They are not soluble in hydrochloric acid nor in ether. [Illustration: FIG. 35.--Leucin spheres and tyrosin needles (Stengel).] _Tyrosin_ crystallizes in very fine colorless needles, usually arranged in sheaves, with a marked constriction at the middle (Fig. 35). It is soluble in ammonia and hydrochloric acid, but not in acetic acid. [Illustration: FIG. 36.--Cystin crystals from urine of patient with cystin calculus (X250).]