_Prevention._--Prevention of calculus especially demands this supply of water and watery rations on all soils and in all conditions in which there is a predisposition to the disease. It must also be sought by attempts to obviate all those conditions mentioned above as causative of the malady. Sometimes good rain water can be furnished in limestone districts, but putrid or bad-smelling rain water is to be avoided as probably more injurious than that from the limestone. Unsuccessful attempts have been made to dissolve calculi by alkaline salts and mineral acids, respectively, but their failure as a remedy does not necessarily condemn them as preventives. One dram of caustic potash or of hydrochloric acid may be given daily in the drinking water. In diametrically opposite ways these attack and decompose the less soluble salts and form new ones which are more soluble and therefore little disposed to precipitate in the solid form. Both are beneficial as increasing the secretion of urine. In cases in which the diet has been too highly charged with phosphates (wheat bran, etc.), these aliments must be restricted and water allowed ad libitum. If the crystals passed with the urine are the sharp angular (octahedral) ones of oxalate of lime, then the breathing should be made more active by exercise, and any disease of the lungs subjected to appropriate treatment. If the crystals are triangular prisms of ammonia-magnesium phosphate or starlike forms with feathery rays, the indications are to withhold the feed or water that abounds in magnesia and check the fermentation in the urine by attempts to destroy its bacteria. In the latter direction plenty of pure-water diuretics, and a daily dose of oil of turpentine in milk, or a dose thrice a day of a solution containing one-tenth grain each of biniodid of mercury and iodid of potassium would be indicated. In considering the subject of prevention, it must never be forgotten that any disease of a distant organ which determines the passage from the blood into the urine of albumin or any other colloid (uncrystallizable) body is strongly provocative of calculus, and should, if possible, be corrected. Apart from cases from geological formation, faulty feeding, and other causes, the grand preventive of calculus is a long, summer's pasturage of succulent grasses, or in winter a diet of ensilage or other succulent feed. The calculi formed in part of silica demand special notice. This agent is secreted in the urine in the form of silicate of potash and is thrown down as insoluble silica when a stronger acid displaces it by combining with the potash to its exclusion. In cases of siliceous calculi, accordingly, the appropriate chemical prevention is caustic potash, which being present in the free state would attract to itself any free acid and leave the silica in its soluble condition as silicate of potash. STONE IN THE BLADDER (VESICAL CALCULUS, OR URETHRAL CALCULUS).