The Mixed Or Tribasic Phosphatic Calculus is very common, at least many calculi are partly formed of phosphates, although few are so entirely. The phosphates are deposited from alkaline urine as a light, bulky, white substance, which is commonly very brittle. The salts are insoluble in water and alkalies, but are very soluble in acids. When a fragment is heated in the blow-pipe flame, the salts melt and form a hard enamel; hence this form is often called the Fusible calculus.