(_b_) _Acid Calcium Phosphate Crystals_.--In feebly add, neutral, or feebly alkaline urines acid calcium phosphate, wrongly called "neutral calcium phosphate," is not infrequently deposited in the form of colorless prisms {111} arranged in stars and rosets (Fig. 38, 1). The individual prisms are usually slender, with one beveled, wedge-like end, but are sometimes needle-like. They may sometimes take forms resembling tyrosin (Fig. 38, 2), calcium sulphate, or hippuric acid, but are readily distinguished by their solubility in acetic acid. Calcium phosphate often forms large, thin, irregular, usually granular, colorless plates, which are easily recognized (Fig. 38, 3). [Illustration: FIG. 38.--Crystals of calcium phosphate: 1, Common form (copied from Rieder's Atlas); 2, needles resembling tyrosin (drawn from nature); 3, large, irregular plates (from nature).] (_c_) _Amorphous Phosphates_.--The earthy phosphates are thrown out of solution in most alkaline and many neutral urines as a white, amorphous sediment, which may be mistaken for pus macroscopically. Under the microscope the sediment is seen to consist of numerous colorless granules, distinguished from amorphous urates by their color, their solubility in acetic acid, and the reaction of the urine. The various phosphatic deposits frequently occur together. They are sometimes due to excessive excretion of phosphoric acid, but usually merely indicate that the urine has become, or is becoming, alkaline. {112} (2) Calcium carbonate may sometimes be mingled with the phosphate deposits, usually as amorphous granules, or, more rarely, as colorless spheres and dumb-bells, Fig. 39, which are soluble in acetic acid with gas-formation. [Illustration: FIG. 39.--Indistinct crystalline sediment (dumb-bell crystals) of calcium carbonate. Similar crystals are formed by calcium oxalate and calcium sulphate (after Funke).] (3) Ammonium Urate Crystals.--This is the only urate deposited in alkaline urine. It forms opaque yellow crystals, usually in the form of spheres (Plate IV, and Fig. 63), which are often covered with fine or coarse spicules, "thorn-apple crystals." Sometimes dumb-bells, compact sheaves of fine needles, and irregular rhizome forms are seen (Fig. 40). Upon addition of acetic acid they dissolve, and rhombic plates of uric acid appear. [Illustration: FIG. 40.--Crystals of ammonium urate (one-half of the forms copied from Rieder's Atlas, the others from nature).] [Illustration: PLATE IV. Sediment of alkaline fermentation (after Hofmann and Ultzmann).] {113} These crystals occur only when ammonia is present in excess. They are generally found along with the phosphates in decomposing urine and have no clinical significance. B. ORGANIZED SEDIMENTS